First of all, you don't need to use anything other than AppGameKit for your app development. What you need from the Google side to get ads running is the admob ID of the ad you've created for your app (format: ca-app-pub-XXXXXXXXXXXXXXXXX).
AGK wise, these commands are relevant to you: ShowFullscreenAdvertAdMob, SetAdMobTesting and SetAdMobDetails. Then it's simply a case of calling them once at the start of your app, ideally in a function, like so:
If you set SetAdMobTesting to 1, you will receive test ads. This is required, if you want to test the app yourself or click on the ads for testing purposes and is required
in such circumstances. Otherwise Google can terminate your ads. It has to be called before calling SetAdMobDetails. When you want to show your ad, you use ShowFullscreenAdvertAdMob if it's a full screen ad. There are other forms of ads supported, too with their respective commands. That's it.