It's very simple.  While at your Home Screen (My Selections), tap on the blue star icon on the top right.  From this menu, you can either search for a team or league.  You can accomplish this by searching outright using the top line or clicking the blue + symbols to search and add specific teams and leagues.  After typing in the name of the team or league, you can choose to view it before you add it or just add it immediately.  Viewing it by tapping its icon may help confirm it's the team you're seeking.   Add the team to your Home Screen by tapping the blue + symbol.