How to do this in normal Fallout 3: After you are able to enter Citadel (After Waters of Life), find Elder Lyons, ask him to grant power armor training, then go find Paladin Gunny, who is usually in the training area during day, ask him to give you Power Armor training and you will gain the Power Armor Training perk. You will now permanently be able to use any type of Power armor. But if you complete The American Dream quest, you will also gain this perk, expect you are in the very end of the game, so I suggest you do it the normal way.
If you have Operation: Anchorage DLC installed, complete the simulation. After you have completed the simulation, you gain Power Armor Training, so you can wear the Power Armor. If not, do the normal Fallout 3.
If you have the PC version, are able to use the console, and feel like cheating, open the console and type "player.addperk 00058FDF". This will instantly give you the perk, wherever you are.