How to be REALLY good at MW2?

I am really into the game, but sometimes I just get really frustrated with it. Some matches I'll get 20+ kills, one match I got like 46... But mist of the time I'll get around 15 Kills and over 20 deaths. How can I be good enough to get killstreaks like 7,8, and 11? Any tipsmon classes or anything else would be nice. Thanks a bunch! My PSN code is ffej4