The Brotherhood of Steel is back in Fallout 4 and here’s a quick guide for you to join it. Although being around in the early stages of the game, we recommend you not to embark on the quest leading to the Brotherhood before your character is at level 10.
Then, getting close to the Brotherhood you will notice a radio transmission and a quest called Call to Arms will pop up. Listen to the radio transmission reporting a help quest coming from the Cambridge police station nearby. Once there you will have to fight a few ghouls and meet Paladin Danse.
At this point, after you talk to him, enter the police station and loot as much as you can (mainly ammo) so you’re at your maximum fire power possible. Talk again to Danse, he will lead you to a factory where you will have to face Synths. The most difficult parts here are opening the doors and getting the power back on: for the doors, use the assistant’s terminal to reset the password; for the power, use the terminal in the back and activate the backup generators (don’t forget the fusion core).
After that you will have to kill the synth incoming (use the engine to torch them) and talk to Danse in order to complete the quest and get the item he will reward you. Then, choose if join the Brotherhood: in case you do, go to the police station for more quests.