Once you go through project purity with the scientists, save them, when at the Citadel ask Elder Lyons if he will let you receive power armor training. Go to the drill sergeant training the Brotherhood shooting the guns and ask him for training. You can also just download operation: Anchorage and you recive Winterized T-21b Power Armor and the perk. (The normal T-21b from You gotta Shoot Em in the head is better until it looses its condition, cant repair either one. The Winterized T-21b hasnt degraded 1% since i've got it and i have played for 20 hrs or so with it on. Or you could get it via Operation:Anchorage.