First of all what you need to do, when the User logs in, Carry the Username in the Session variable like this
//if the user is Successfully logged in
Session["Username"] = txtusername.text;
and in all your other pages , you will probably have a Label that will Display the name of the User who is currently logged in , you would get the value from the Session like this
lblUser.Text = Session["Username"].Tostring();
And when your user logout of your Session then you need to abandon the session or set it to null
Session["Username"] = null
Hope this Helps
Regards
Vuyiswa Maseko