G AdMob (by Google) is certainly one of the most used advertising solution for mobile apps.

You will some fields to fill:

• Publisher ID. Add your Publisher ID Click Here to know how to find your Publisher ID.
• Ad Unit ID. Add your Ad Unit ID Click Here to know how to find your Ad Unit ID.
• Ads Position. Let you control Ads Position (Top Center, Bottom Center, Center).
• Ads Size. Let you control Ads Size (Smart Banner, Banner, Medium Rectangle, Full Banner, Leaderboard, Skyscraper).

Update your setting and finally, Press "Save Changes" button.