As @yugidroid, your code is pretty messy! :P But you will get it eventually, don't worry!
Most important thing, don't put layouts on your manifest, they don't go there!
In your folder /layout (inside your project) create a new xml file called "activity_main.xml" and fill it with this:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:id="@+id/linearLayout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"></LinearLayout>
</RelativeLayout>
On your java file, change to
adView.setAdUnitId("xx-xxx-xxx-1234567890123456/1234567890"); //Change the numbers to what google sent you in the email!!!
and Adrequest change to
AdRequest adRequest = new AdRequest.Builder().build();
(not recommended for testing but for now it will do)
I recommend you though to read the documentation again though, it is pretty straight forward
https://developers.google.com/mobile-ads-sdk/docs/#play
https://developers.google.com/mobile-ads-sdk/docs/admob/fundamentals#play