Add a new site to track in Google Analytics
Click Admin in the top menu
Account -> Property -> view
if it's a new subdomain of a domain you already track
- pick the account (i.e. domain) and for property: "new property" (you get 50 of these per accoutn)
Or for a whole new domain:
- under Admin -> Account "Create new account" (you get 100 of these)
When adding the new view, it will default the name to "All Website Data" but instead name it after your exact subdomain.
May also want to add it in 'Google Search Console' (formerly 'Google Webmaster Tools')
//TODO: add site in Google Search Console