Become an initiate

Become an initiate is a side quest in Fallout.

Detailed walkthrough
When first approaching the entrance to the Brotherhood of Steel bunker at Lost Hills, Cabbot will greet the Vault Dweller in an unusual manner. If asked about joining the Brotherhood, he explains that anyone who wants to join the Brotherhood must first complete a quest, an order from high elder Maxson. The quest involves retrieving an ancient Brotherhood disk from the "ancient order."
 * If the Vault Dweller's Intelligence is 3 or less, they cannot complete this quest, as they can't ask Cabbot about joining. Darrel, the paladin standing to his left, will laugh at the Vault Dweller, stating that the elders instated a minimum IQ requirement for new initiates. He will, however, point the Vault Dweller to the Glow, telling them to search the facility to obtain better arms. He will also give them some Rad-X, warning about the high radiation levels present there. Even if the holodisk is retrieved, the Vault Dweller will not have the option to give it to Cabbot.

The surface and the underground facility at the Glow are both highly irradiated, so anti-radiation chems are a necessity:
 * Rad-X can be bought from Vance or from Mitch in the All-n-One Store in the Hub. Alternatively, six of them can be found in an ice chest in the Boneyard Library.
 * RadAway can also be bought from Vance. It is useful, as the Vault Dweller might accumulate some radiation before getting the chance to use Rad-X. Use caution, as it is possible to become addicted.
 * Both of the above can also be bought in the Cathedral.

A rope is also necessary to get inside:
 * There are two in Shady Sands:
 * Seth possesses a rope that can be bartered for, or stolen.
 * One can be found in the eastern portion of the town in the house next to the crops.
 * Mitch has five ropes to trade for.

Once all the necessary supplies have been gathered, travel to the Glow. One needs to take two Rad-X as soon as they arrive to gain 100% resistance to radiation since the whole area is heavily irradiated. They then need to use the rope on the metal beam protruding into the crater on the left side to descend down into the facility.

The holodisk is located on a corpse wearing power armor. The Vault Dweller needs to circle the level to get it. Once it is in the Vault Dweller's possession, delivering it to Cabbot will complete the quest.

Rewards

 * 2000 XP.
 * Becoming an initiate of the Brotherhood of Steel and gaining access to Lost Hills.