If you are talking about Fallout 3, then you have to do a lot of things before you can be trained to where the armor. What you need to do is find the Brotherhood of Steel Citadel (I can't remember how to get there because I haven't played the game in awhile) and you have to speak to a Brotherhood of Steel member to get trained. A tip though, when you get trained, put on the Enclave armor because it has more resistance against radiation and lasts a lot longer. After that you will come along armor called "Tesla Armor". Pick it up, put it on, and drop your other armor. Tesla Armor is incredibly strong and can withstand a lot of damage.