If you want to track your site using Google Analytics, you can easily do so by:
1- Going to the Settings page
2- Entering your Google Analytics ID (following this format 'UA-XXXXXXXX-X')
3- Pressing "Save", and that's it!
Note: Entering the Google Analytics ID allows the tracking code to be automatically generated by the website, so there is no need to enter it manually inside the custom scripts area.
If you don't have a Google Analytics Account go ahead and setup an account: https://support.google.com/sites/answer/97459?hl=en
If you already have a Google Analytics Account but don't know how to get your site ID, follow this tutorial: https://support.google.com/analytics/answer/1032385?hl=en