In Fallout 3, you get to wear Power Armor once you've completed enough of the main quest-line to be allowed to TRAIN the perk Power Armor at the Brotherhood of Steel Main base, The Citadel. So, get on with the quests, get to the Citadel via the main quest-line and the find the training Sergeant, and pester him about the shiny Power Armor.
Remember to loot all the armor off of dead BOS-soldiers and Paladins you find/see around the DC area. Also Enclave soldiers are a neat source of Power Armor.
Or you can Glitch in the Citadel.
Jeez, Just play through the game and you'll get the Power Armor Training.