If you don't have an AdMob account, go to AdMob to sign up.
You are set if you are navigated to the screen below
AdMob page
Creating an AdMob unit
An ad unit is one or more Google ads displayed using one piece of the AdSense ad code. You need to generate an Ad Unit to show ads inside your application.
Create an Ad Unit inside your AdMob account
Under your AdMob console, click on Apps and click on Add App. Follow the steps to add the app.
Enter an app name, select Android, and click Add
Adding an app not on the Play Store
On the next page, click Next: Create Add Unit
You don't need the App ID for Sketchware
On the next page, click either Banner or Interstitial
You can specify ad type (Text, Image, and rich media or Video), and more under Advanced Settings.
Enter an ad unit name, and click Create Ad Unit
Ignore the steps on the next screen and copy the ad unit ID (the one at the bottom). You will need it for the next step.