To get a Google Analytics key for your site, go to "https://analytics.google.com". There, sign in with your Google ID and once signed in, you'll see this page.
Click on Admin.
Then, add a new property by clicking "Create property".
Then, give your website a name, add your URL, choose a category for your site and then click "Get Tracking ID".
Then, you'll see a Tracking ID for your site. Copy it and add to the "Google Analytics" key on the Settings page.