First, you need to create an account on Google Analytics and add your website to Google Analytics. See here how you can do this.
1: Log on to your Site Builder
2: Go to Site settings
3: Click on Google Analytics
4: Now find your Tracking-ID, you can find your unique Tracking ID at Google Analytics when you have added your website to Google Analytics.
5: Enter your Tracking ID to your Site Builder
6: Now click on the Save button, you can find this on the bottom in the right corner of your page