BROTHERHOOD OF STEEL QUEST 01 - Reveille
Triggered near the end of Story 06 - Reunions when you emerge from Fort Hagan and see the Prydwyn airship flying overhead.
Note: This quest can be bypassed (deliberately or by accident) if you get within range of the radio transmitter at Cambridge police station (military frequency AF95). You will advance to the BoS quest 02 - Fire support. If you continue on with the Brotherhood quests past the point where you officially join the brotherhood, BoS quest 01 - Reveille will no longer trigger. There is no penalty for this, but you will miss out on the XP from this easy quest.
Objective: Investigate the Brotherhood of Steel airship
After seeing the massive airship fly overhead at Fort Hagen, you'll have two options. The quest marker will point you towards Boston Airport which is a fairly difficult area to reach safely unless you have the aquaboy/ girl perk, allowing you to swim around to reach it. The other option is to head straight for Cambridge police station which bypasses this objective and takes you straight to the second Brotherhood of Steel quest. If you do visit the airport, talk to the brotherhood knight at the main entrance to find out about the airship and get some info about joining the brotherhood's ranks. Turns out you'll need to visit the Cambridge police station. Yup, what a time-sink this trip was.
Objective: Proceed to Cambridge police station
Note: The first and second brotherhood quests can overlap, meaning you have both active at the same time.
The Cambridge police station (if you don't have it on your map yet) is directly South of the Corvega Assembly Plant. Or to the North-West of Diamond City. Once you get close enough, the radio signal should trigger the Brotherhood of Steel quest 02 - Fire Support, with a marker pointing to the police station. When you get close to the outskirts of Cambridge, the quest will come to an abrupt end, leaving you to carry on investigating the police station for BoS quest 02 - Fire Support.