I have been working on this program for days and I can not figure out what I am doing wrong. I have most of the code done but I am getting error messages with the radio buttons. The new user radio button is supposed to take the data inserted in the textboxes and display it in the second listbox but it only does this with the first input. How can I fix this? The existing user radio button is supposed to allow me to select a line from the second listbox and then it should split the line and add the data to the three textboxes. I throws an exception error when I click the existing user radio button. How can I correct this error? This is the code I have so far. I am going to post the entire code just in case it is needed.

Private Sub btnProcess_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnProcess.Click
    'I am dimming my variables that I need 
    Dim name As String
    Dim address As String
    Dim city As String
    Dim firstlast As String
    Dim invoicenumber As String
    Dim chairs As Double
    Dim sofas As Double

    'I am assigning value to my variables
    name = txtName.Text
    address = txtAddress.Text
    city = txtCity.Text
    chairs = CDbl(txtChairs.Text) * 350
    sofas = CDbl(txtSofas.Text) * 925

    'I am using if clauses to make sure the user puts values into the text boxes and if not then a message box will appear
    If name = "" Then
        MessageBox.Show("Please Enter Your Name.")
        txtName.Focus()
    End If
    If address = "" Then
        MessageBox.Show("Please Enter Your Address.")
        txtAddress.Focus()
    End If
    If city = "" Then
        MessageBox.Show("Please Enter Your City, State, and Zip in Correct Format.")
        txtCity.Focus()
    End If
    If address.Contains(",") = False Then
        MessageBox.Show("Please Enter In Correct Format:  City, St., Zip.")
    End If

    'These are my functions that I made for the invoice number and name reverse
    invoicenumber = GenerateinvoiceNumber(txtName.Text)
    firstlast = RevName(name)

    'I am adding all of the items to my listbox
    lstResults.Items.Clear()
    lstResults.Items.Add("INVOICE NUMBER: " & " " & invoicenumber.ToUpper)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Name: " & "   " & RevName(txtName.Text.ToUpper))
    lstResults.Items.Add("Address: " & "" & address.ToUpper)
    lstResults.Items.Add("City: " & "       " & city.ToUpper)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Number of Chairs: " & "" & txtChairs.Text)
    lstResults.Items.Add("Number of Sofas: " & " " & txtSofas.Text)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Cost: " & "           " & FormatCurrency(chairs + sofas))
    lstResults.Items.Add("Sales Tax:" & "   " & FormatCurrency((chairs + sofas) * 0.05))
    lstResults.Items.Add("--------------------------------------------")
    lstResults.Items.Add("Total Cost: " & " " & FormatCurrency((chairs + sofas) * 1.05))

End Sub
'This is the function for the name reverse. 
Function RevName(ByVal name As String) As String

    Dim firstlast() As String

    'This split is to take the part of the word after the comma
    firstlast = Split(txtName.Text.ToUpper, ", ")
    'This if clause is to make sure that the name is formatted correctly with a comma
    If name.Contains(", ") = False Then
        MessageBox.Show("Please Enter Your Name In Correct Format.")
    End If

    'This will show the function and the correct formatting in my list box
    Return firstlast(1) & " " & firstlast(0)

End Function
'This is my function to create the invoice number
Function GenerateinvoiceNumber(ByVal s As String) As String
    'Dimming the variables I need for my function
    Dim initials() As String
    Dim firstInitial, invoicenumber, city As String

    'Assigning values to the variables
    'I am using a split to get the letters from the last name
    initials = s.Split(CChar(","))
    'I am taking the letters that I need using a substring
    firstInitial = initials(0).Trim.Substring(0, 2).ToUpper

    city = txtCity.Text
    'This is to get the value of the textbox city and the length of the input
    Dim length = city.Length
    'This is to get the first two letters of the name behind the comma and the city substring will extract the last four characters in the city textbox
    invoicenumber = (firstInitial & city.Substring(length - 4, 4))
    'This is to return the value of the function
    Return invoicenumber
End Function
Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click
    'This is to clear the form and start over
    txtName.Clear()
    txtAddress.Clear()
    txtCity.Clear()
    txtChairs.Clear()
    txtSofas.Clear()
    lstResults.Items.Clear()

End Sub

Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
    'This is to end the entire form
    Me.Close()
End Sub

This is the Part I am having trouble with

*** Private Sub rbtnNew_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnNew.CheckedChanged

    If rbtnNew.Checked Then rbtnNew.Enabled = True Else rbtnExisting.Enabled = False

    If rbtnNew.Checked = True Then
        lstCustomer.Items.Add(RevName(txtName.Text.ToUpper) & " , " & (txtAddress.Text.ToUpper) & " , " & (txtCity.Text.ToUpper))
    End If

End Sub

Private Sub rbtnExisting_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnExisting.CheckedChanged

    Dim output() As String
    Dim listselected As String
    Dim name1, address2, city2 As String
    listselected = CStr(lstCustomer.SelectedItem)
    'I am using a split to get the letters from the last name
    output = listselected.Split(CChar(","))

    'Assigning values to the variables
    name1 = txtName.Text
    address2 = txtAddress.Text
    city2 = txtCity.Text

    'I am taking the letters that I need using a substring
    txtName.Text = output(0).Trim.Substring(CInt(","))
    txtAddress.Text = output(1).Trim.Substring(CInt(","))
    txtCity.Text = output(2).Trim.Substring(CInt(","))

    If rbtnExisting.Checked Then rbtnExisting.Enabled = True Else rbtnNew.Enabled = False

    lstResults.Items.Clear()
    txtChairs.Clear()
    txtSofas.Clear()

End Sub***

End Class

There is a convention in writing programming codification is Check the validation before assigning the values to the variables and force the user to input a value unless it is optional.
But,here you do that from reverse. First you have assigned the values to the variables then checked the validation.
I do here a function to check the validation. Remove the lines 19 to 33 and call the function before line no 11.

 Private Function ValidatingText() As Boolean

        If String.IsNullOrEmpty(txtName.Text) Then
            MessageBox.Show("Please Enter Your Name.")
            txtName.Focus()
            Return False
        End If
        If String.IsNullOrEmpty(txtAddress.Text) Then
            MessageBox.Show("Please Enter Your Address.")
            txtAddress.Focus()
            Return False
        End If
        If String.IsNullOrEmpty(txtCity.Text) Then
            MessageBox.Show("Please Enter Your City, State, and Zip in Correct Format.")
            txtCity.Focus()
            Return False
        End If

        Return True
    End Function

Put the following codes at the line no 11 to check the input validation.

If ValidatingText() = False Then
            Exit Sub
        End If

But I do not understand the motif or the conception of the following.

If rbtnNew.Checked Then rbtnNew.Enabled = True Else rbtnExisting.Enabled = False

Well, now this is time to go to my workig place. I will try to solve the problem in the evening.

This is most interesting that you have forced the user to seperate the every word of his name by a ",", like "FirstName","MiddleName","SurName".

Leave them free and split the text by a space.
Like this

Function RevName(ByVal name As String) As String
    Dim firstlast() As String
    Dim RevName as String=""

    'This split is to take the part of the word after the comma
    firstlast = Split(name.ToUpper, " ")

    'Now join the name reversely
    For i As Integer = Ubound(firstlast) To 0 Step -1
        RevName &=firstlast(i).Trim & " "
    Next

    Return RevName.Trim
End Function

Suppose it will help you.

Thank You and the comma is required by the professor, I have done this now. I had changed a few things before you responded and this function works perfectly I really appreciate this. I still cant get the radio buttons to work. I cant select the line in the listbox and add the data to the textboxes.

Private Sub btnProcess_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnProcess.Click

   'I am dimming my variables that I need 
    Dim name As String
    Dim address As String
    Dim city As String
    Dim firstlast As String
    Dim invoicenumber As String
    Dim chairs As Double
    Dim sofas As Double

    If ValidatingText() = False Then
        Exit Sub
    End If
    'I am assigning value to my variables
    name = txtName.Text
    address = txtAddress.Text
    city = txtCity.Text
    chairs = CDbl(txtChairs.Text) * 350
    sofas = CDbl(txtSofas.Text) * 925

    'I am using if clauses to make sure the user puts values into the text boxes and if not then a message box will appear


    'These are my functions that I made for the invoice number and name reverse
    invoicenumber = GenerateinvoiceNumber(txtName.Text)
    firstlast = RevName(name)

    'I am adding all of the items to my listbox
    lstResults.Items.Clear()
    lstResults.Items.Add("INVOICE NUMBER: " & " " & invoicenumber.ToUpper)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Name: " & "   " & RevName(txtName.Text.ToUpper))
    lstResults.Items.Add("Address: " & "" & address.ToUpper)
    lstResults.Items.Add("City: " & "       " & city.ToUpper)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Number of Chairs: " & "" & txtChairs.Text)
    lstResults.Items.Add("Number of Sofas: " & " " & txtSofas.Text)
    lstResults.Items.Add(String.Empty)
    lstResults.Items.Add("Cost: " & "           " & FormatCurrency(chairs + sofas))
    lstResults.Items.Add("Sales Tax:" & "   " & FormatCurrency((chairs + sofas) * 0.05))
    lstResults.Items.Add("-------------------------------------------------------------")
    lstResults.Items.Add("Total Cost: " & " " & FormatCurrency((chairs + sofas) * 1.05))

End Sub
'This is the function for the name reverse. 
Function RevName(ByVal name As String) As String

    Dim firstlast() As String

    'This split is to take the part of the word after the comma
    firstlast = Split(txtName.Text.ToUpper, ", ")
    'This if clause is to make sure that the name is formatted correctly with a comma
    If name.Contains(", ") = False Then
        MessageBox.Show("Please Enter Your Name In Correct Format.")
    End If

    'This will show the function and the correct formatting in my list box
    Return firstlast(1) & " " & firstlast(0)

End Function
'This is my function to create the invoice number
Function GenerateinvoiceNumber(ByVal s As String) As String
    'Dimming the variables I need for my function
    Dim initials() As String
    Dim firstInitial, invoicenumber, city As String

    'Assigning values to the variables
    'I am using a split to get the letters from the last name
    initials = s.Split(CChar(","))
    'I am taking the letters that I need using a substring
    firstInitial = initials(0).Trim.Substring(0, 2).ToUpper

    city = txtCity.Text
    'This is to get the value of the textbox city and the length of the input
    Dim length = city.Length
    'This is to get the first two letters of the name behind the comma and the city substring will extract the last four characters in the city textbox
    invoicenumber = (firstInitial & city.Substring(length - 4, 4))
    'This is to return the value of the function
    Return invoicenumber
End Function
Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click
    'This is to clear the form and start over
    txtName.Clear()
    txtAddress.Clear()
    txtCity.Clear()
    txtChairs.Clear()
    txtSofas.Clear()
    lstResults.Items.Clear()

End Sub

Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
    'This is to end the entire form
    Me.Close()
End Sub

Private Sub rbtnNew_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnNew.CheckedChanged
    ValidatingText()
    If rbtnNew.Checked = Enabled Then
        rbtnExisting.Checked = False
    End If

    If rbtnNew.Checked = True Then
        lstCustomer.Items.Add(RevName(txtName.Text.ToUpper) & " , " & (txtAddress.Text.ToUpper) & " , " & (txtCity.Text.ToUpper))
    End If

End Sub

Private Sub rbtnExisting_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnExisting.CheckedChanged
     rbtnExisting.Checked = Enabled 

    txtName.Text = CStr(Enabled)
    txtAddress.Text = CStr(Enabled)
    txtCity.Text = CStr(Enabled)


    Dim output() As String
    Dim listselected As String
    Dim name1, address2, city2 As String
    listselected = CStr(lstCustomer.SelectedItem)

    lstResults.Items.Clear()
    txtName.Clear()
    txtAddress.Clear()
    txtCity.Clear()
    txtChairs.Clear()
    txtSofas.Clear()

    'Assigning values to the variables
    name1 = txtName.Text
    address2 = txtAddress.Text
    city2 = txtCity.Text

    If rbtnExisting.Checked = True Then
        'I am using a split to get the data from the listbox to split into the textboxes
        output = listselected.Split(CChar(","))
        'I am taking the data that I need using a substring and adding to the textboxes
        txtName.Text = output(0).Trim.Substring(CInt(","))
        txtAddress.Text = output(1).Trim.Substring(CInt(","))
        txtCity.Text = output(2).Trim.Substring(CInt(","))
    End If


End Sub
Private Function ValidatingText() As Boolean
    If String.IsNullOrEmpty(txtName.Text) Then
        MessageBox.Show("Please Enter Your Name.")
        txtName.Focus()
        Return False
    End If
    If String.IsNullOrEmpty(txtAddress.Text) Then
        MessageBox.Show("Please Enter Your Address.")
        txtAddress.Focus()
        Return False
    End If
    If String.IsNullOrEmpty(txtCity.Text) Then
        MessageBox.Show("Please Enter Your City, State, and Zip in Correct Format.")
        txtCity.Focus()
        Return False
    End If
    Return True
End Function

End Class

@LoveAntwinette

In your codification there are too many mistakes you did. You use CDbl() Fuunction to convert the value in txtChairs and txtSofas TextBoxes to Numeric.
But CDbl() never converts a String Type Value To a Double Type Numerical Value.
To covert from String To Numerical Value (It should convert to be Double or Integer Type) you would be used the Val() Function.
Like

chairs = CDbl(Val(txtChairs.Text)) * 35
sofas = CDbl(Val(txtSofas.Text)) * 925

By using Val() Function you can get the numerical part of an alphanumeric string type value. But it supplies only that numeric value which appears first in a string. Like

Val("1txt")=1
Val("10txt")=10
Val("t1xt")=1
Val("t10xt")=10
Val("t1x0t")=1

There are a massive mistake in your codes
If rbtnExisting.Checked = True Then
'I am using a split to get the data from the listbox to split into the textboxes
output = listselected.Split(CChar(","))
'I am taking the data that I need using a substring and adding to the textboxes
txtName.Text = output(0).Trim.Substring(CInt(","))
txtAddress.Text = output(1).Trim.Substring(CInt(","))
txtCity.Text = output(2).Trim.Substring(CInt(","))
End If

Here you already split the string by a ',' i.e. output = listselected.Split(CChar(",")). So, why did you try to eleminate the ',' from the array element. i.e. txtName.Text = output(0).Trim.Substring(CInt(",")). There is no need to do that, you already eleminate the ',' when you split it on that character.
I do some modification in your codes after reading it. It should help you.

Public Class Form1

    'I am dimming my variables that I need 

    Dim address As String
    Dim city As String
    Dim firstlast As String
    Dim invoicenumber As String
    Dim chairs As Double
    Dim sofas As Double

    Private Sub btnProcess_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnProcess.Click
        If ValidatingText() = False Then
            Exit Sub
        End If




        name = txtName.Text
        address = txtAddress.Text
        city = txtCity.Text
        chairs = CDbl(Val(txtChairs.Text)) * 350
        sofas = CDbl(Val(txtSofas.Text)) * 925

        invoicenumber = GenerateinvoiceNumber(txtName.Text)
        firstlast = RevName(name)

        lstResults.Items.Clear()
        lstResults.Items.Add("INVOICE NUMBER: " & " " & invoicenumber.ToUpper)
        lstResults.Items.Add(String.Empty)
        lstResults.Items.Add("Name: " & "   " & RevName(txtName.Text.ToUpper))
        lstResults.Items.Add("Address: " & "" & address.ToUpper)
        lstResults.Items.Add("City: " & "       " & city.ToUpper)
        lstResults.Items.Add(String.Empty)
        lstResults.Items.Add("Number of Chairs: " & "" & txtChairs.Text)
        lstResults.Items.Add("Number of Sofas: " & " " & txtSofas.Text)
        lstResults.Items.Add(String.Empty)
        lstResults.Items.Add("Cost: " & "           " & FormatCurrency(chairs + sofas))
        lstResults.Items.Add("Sales Tax:" & "   " & FormatCurrency((chairs + sofas) * 0.05))
        lstResults.Items.Add("-------------------------------------------------------------")
        lstResults.Items.Add("Total Cost: " & " " & FormatCurrency((chairs + sofas) * 1.05))
    End Sub
    'This is the function for the name reverse. 
    Function RevName(ByVal name As String) As String
        Dim firstlast() As String
        Dim RName As String = ""
        'This split is to take the part of the word after the comma
        firstlast = Split(name.ToUpper, ",")
        'Now join the name reversely
        For i As Integer = Ubound(firstlast) To 0 Step -1
            RName &= firstlast(i).Trim
            If i > 0 Then
                RName &= " "
            End If
        Next

        Return RName.Trim
    End Function
    'This is my function to create the invoice number
    Private Function GenerateinvoiceNumber(ByVal s As String) As String

        Dim initials() As String
        Dim firstInitial, invoicenumber, city As String

        initials = s.Split(CChar(","))
        'I am taking the letters that I need using a substring
        firstInitial = initials(0).Trim.Substring(0, 2).ToUpper
        city = txtCity.Text
        'This is to get the value of the textbox city and the length of the input
        Dim length = city.Length
        'This is to get the first two letters of the name behind the comma and the city substring will extract the last four characters in the city textbox
        invoicenumber = (firstInitial & city.Substring(length - 4, 4))
        'This is to return the value of the function
        Return invoicenumber
    End Function
    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click
        'This is to clear the form and start over
        txtName.Clear()
        txtAddress.Clear()
        txtCity.Clear()
        txtChairs.Clear()
        txtSofas.Clear()
        lstResults.Items.Clear()
    End Sub
    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
        'This is to end the entire form
        Me.Close()
    End Sub
    Private Sub rbtnNew_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnNew.CheckedChanged

        If rbtnNew.Checked Then

            If Not ValidatingText() Then
                rbtnNew.Checked = False
                Exit Sub
            End If

            lstCustomer.Items.Add(RevName(txtName.Text.ToUpper) & " , " & (txtAddress.Text.ToUpper) & " , " & (txtCity.Text.ToUpper))
        End If
    End Sub
    Private Sub rbtnExisting_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rbtnExisting.CheckedChanged


        Dim output() As String
        Dim name() As String
        Dim listselected As String

        'If selection is 0 then exit from procedure
        If lstCustomer.SelectedItems.Count = 0 Then Exit Sub

        listselected = CStr(lstCustomer.SelectedItem)

        'To Clear the TextBoxes call the Click event of the Button btnClear
        btnClear.PerformClick()


        If rbtnExisting.Checked Then
            'split and store in an array
            output = listselected.Split(CChar(","))

            'adding to the textboxes


            'Transferring the name as input format i.e. "First_Name, Middle_Name, Last_Name"
            name = output(0).Trim.Split(CChar(" "))

            For i As Integer = UBound(name) To 0 Step -1
                txtName.Text &= name(i)
                If i > 0 Then
                    txtName.Text &= ", "
                End If
            Next


            txtAddress.Text = output(1)

            'Transferring the city as input format i.e. "First_Name, Middle_Name, Last_Name"
            For i As Integer = 2 To UBound(output)
                txtCity.Text &= output(i)
                If i < UBound(output) Then
                    txtCity.Text &= ", "
                End If
            Next
        End If
    End Sub
    Private Function ValidatingText() As Boolean

        'First Checking Name TextBox is Empty or not
        If String.IsNullOrEmpty(txtName.Text) Then

            'If Empty Prompt to the user
            MessageBox.Show("Please Enter Your Name.")
            txtName.Focus()
            Return False

        Else
            'If Not Empty then Check for it contains ","
            If Not txtName.Text.Contains(",") Then
                'If Not Prompt to the User
                Dim msgx As String = String.Format("Please, input your name in correct format.{0}Like : '{1}, {2}, {3}'", vbCrLf, "First_Name", "Middle_Name", "Last_Name")
                MessageBox.Show(msgx)
                txtName.Focus()
                txtName.SelectAll()
                Return False
            End If

        End If

        'Ckeck for Address is empty or not
        If String.IsNullOrEmpty(txtAddress.Text) Then
            MessageBox.Show("Please Enter Your Address.")
            txtAddress.Focus()
            Return False
        End If


        'Check City is Empty or Not
        If String.IsNullOrEmpty(txtCity.Text) Then
            'If vbEmpty Prompt to the User
            MessageBox.Show("Please Enter Your City, State, and Zip Code.")
            txtCity.Focus()
            Return False
        Else
            'Check it contains ","
            If Not txtCity.Text.Contains(",") Then
                'If Not Prompt to the User
                Dim msgx As String = String.Format("Please, input your City, State, and Zip in correct format.{0}Like : '{1}, {2}, {3}'", vbCrLf, "City_Name", "State_Name", "Zip_Code")
                MessageBox.Show(msgx)
                txtCity.Focus()
                txtCity.SelectAll()
                Return False
            End If
        End If

        'If all inputting is Ok 
        Return True

    End Function
End Class
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