Talk to the guard in the south entrance for valuable information about the base. In the north of the base has a small gate for you to leave and enter aswell.
The Lost Hills bunker is the main base of the Brotherhood of Steel in Fonline 3 (And in Fallout as well). You are able to enter it and explore, but dont trespass the inner gate.
This location have many npc's to talk with good quests for beginners.
Lost Hills is located at Grid 26:35 (lower central area, reasonably close to Junktown).