Correct, you have to upload your app to Google Play.
You can set up alpha and beta testers before the real publishing.
Most people use AdMob and Chartboost ads, so you have to set up an account there, which is free and easy.
You then set up a campain and link your app to that campain.
The implementation in AppGameKit is easy, there are a lot of threads here with good example codes.
You can also use test ads before using your own paying ads ( see help ), or use the test details provided in the example projects!
DONT CLICK ON YOUR OWN PAYING ADMOB ADS. DOING SO MAY RESULT IN YOUR ACCOUNT GETTING BANNED!