COD: Black Ops

So, I've played COD Black Ops with my boyfriend a few times within the last few weeks. This game is not something for me it seems. I can't figure out how to play it right. I try staying in corners and shooting at people and they kill me. I run around and they come up behind me and knife me. I throw grenades and they snipe me. Don't even ask if I've been able to deploy an RCXD because obviously you know the answer to that.