To get started you'll need to set up your Google Analytics account. Here are instructions for setting up your Google Analytics account properly.
Once you've created your account, it's time to copy your analytics ID and paste it into the pre-set field in the site builder. In Google analytics, click on the Admin gear in the bottom left hand corner, then under the "Properties" section, click "Tracking Info" and then "Tracking Code":
On that page, all you need to do is copy the "Tracking ID" at the top:
Back in your site builder click the triple bars in the upper left hand corner, and then click settings:
In settings, click over to "Integrations" and paste your copied ID into the "Google Analytics Tracking ID" field.
Click submit, then save your site and republish and your site analytics will start to be recorded into Google Analytics so you can start tracking your visitor data.