To trigger this quest, head to Cambridge, which is south of Lexington. The quest will automatically trigger once you reach Cambridge. Your first objective is to listen to the Military Frequency AF95 on your Pip-Boy. You'll learn that there's a shootout at the Cambridge Police Station.
Expect to find a lot of Ghouls along the way. Take them out for experience and feel free to explore a bit; reaching the police station isn't time-sensitive.
Once you reach the police station, help out the humans against the ghouls approaching the station. Then talk to the Brotherhood leader, Paladin Danse, who will walk back into the police station, triggering the completion of the quest.