Not sure where you are coming from but you would want to get on the orange and/or blue lines in the directions of largo town center and new carrollton if coming from downtown. You want to get off at the stadium armory station. If you do that the exit you want is toward the front of the cars the way the train is going. When you come out of the station you will be along side the armory. Just keep walking straight, past all the bus stops and make a right at the corner.
The station for the DC Armory is called Stadium-Armory.The Orange Blue lines stop there.