Power Armor Training (Fallout 3)

Power Armor Training is a Fallout 3 perk. You must have this perk before you can operate or wear a suit of Power Armor or a Power Helmet. The perk can be gained in two ways: at the Brotherhood of Steel Citadel by Paladin Gunny (which you may access at the beginning of the main quest Picking up the Trail), or by finishing the Anchorage quest in the Operation: Anchorage add-on.

Through the main storyline
After gaining permission from Elder Lyons, speak to Paladin Gunny in the courtyard of the Citadel. (Brotherhood members refer to it as the "bailey", which means the same thing.) There is no charge or other requirement for training - the screen simply fades to black as Gunny begins teaching you. Once that's done, you may use any kind of Power Armor, including the Enclaves' and the Outcasts' varieties.

If you do not ask for training on your own, you will get it automatically at the end of The American Dream when you become an honorary member of Lyons' Pride.

Through Operation: Anchorage
You automatically gain this perk when you finish the Anchorage Reclamation simulation in the Operation: Anchorage add-on.

Console command
Through normal gameplay, training is required to wear power armor of any variety. However, the skill can be unlocked via the console in the PC version by typing. An alternate way to do this is to type in the console.

In New Vegas
Once you finish the quest (INSERT QUEST NAME) for the brotherhood, the Elder will say he is in the process of ending the lockdown. Return to him later (unsure about specifics) and you will be able to ask to join the brotherhood. Doing this will start a quest to install a device in the Black Mountain radio tower. Upon your return Elder McNamara will train you in power armor, give you a suit, and allow you to buy better items from the armorer.