Go to AdMob website and create an account. Then, create a new app from the AdMob interface and an ad unit. Note the identifiers for both the app and the ad unit, as they will be used in your game.
In GDevelop, open the properties of your game and paste the AdMob app id. This acts as the identification that allows AdMob to serve ads in your app.
Add events in your game to load and show the ad when needed (this can be when a level is successfully completed for example). Paste the ad unit identifier into the parameters of the action. You can also enter "test" for loading a test ad.
You can test and publish your game on Android directly from GDevelop. In the editor, open your game and then the Export window. Choose Android, click on Export and wait a few minutes.