You can set up monetized banner ads within your apps using the Ads section. You'll need to register for an AdMob account to start (this account is connected to your Google account).
Note: AdMob only works for Android.
1. Log into your AdMob account and select Get Started.
2. Search for an app to monetize. Then select "Banner." Now you have a few options to customize your banner, and a name to associate with the ad.
3. You'll then see the screen below, with the Ad unit ID in bold. This is the code you'll use to integrate the ads into your app.
4. Copy & paste your unit ID into the Ad Unit ID field of the Ads section, and click Save! And you're done!