imso 0 Junior Poster in Training

How should i reorient an android application in landscape mode once an apps is started?? I started an app but it does not automatically switch to landscape mode.. I tried writing it to "horizontal" but it does not work..

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >