This trophy is attained inside of Diamond City
, which is accessed relatively early on in the main story quest line. You'll notice that Diamond City is a baseball stadium and you'll see a small home plate just at the base of the stairs from where you enter. From the home base, run to first base, then second, then third, and back to home in order to "Get a Homerun", and the trophy will unlock. Note that the bases are numbered and marked as well.