Talk to the guard to be able to become an Initiate

Go to the Lost Hills bunker and talk to the guard to the left of the little bunker. Be careful not to get too close of the door, and to quickly talk to the guard, or he will shoot you. You need to leave all your factions (NPC-factions and Player factions as well) before doing this quest.

Ask the guard about this place and the Brotherhood, then ask him to be granted access inside. He will accept you as a recruit, but only if you bring some items :

• If your Charisma is below 6, you will need to pay 20 AP Rockets.
• If your Charisma is 6 or above, you will only need to pay 120 rounds of 5mm AP.

Bring them to the same guard and talk to him. You are now an Initiate, part of the Brotherhood of Steel !

Rewards

• Joining the Brotherhood of Steel faction (Access to the Lost Hills bunker). You are now a Brotherhood of Steel member !