How do i get better at CoD Modern Warfare 2 online?

So this is my bros game and hes letting me play it now. hes level 70 prestige 10. How do i get better at this game?? everytime i try to shoot someone he ends up like one shotting me. and I look at the killcam of people who kill me and its like they knew i was gonna be there. Im practically camouflaged, and yet they still easily kill me and kill the next person and the next and the next. How do i get better?