I dont understand how to play this game

Im playing with my partner but I feel completely useless. I die super quickly and even on the easiest difficulty I just die so quick. I cant seem to hide and the moment I shoot once and kill one of them I get like 5 guys that shoot me in an instant and kill me. Ive died so far like 25 times attacking one facility. I try to kill them but I die faster than I can react.

What do I do?