In the First mission there is a locked armory to the left if you went in the front door, it contains grenades and a bazooka. To get the key card enter by using the claw on the vent outside, at the end of the catwalk you will drop down and see a guard facing away. Drop in, get right on top of him and hit him with your fist, he should go down in one or two hits and you will get the key card if you're close enough.