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.

Deep in the Old Olney Sewer
Next to the Prototype Medic Power Armor is note that gives you the perk.

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.