MW2 Classes: I need MAJOR HELP?

I need a lot of help with my classes. I am a rank 40, no prestige (de-ranked after getting to 10th 70). I suck terribly at the game and I always go negative. I want to go all MLG pro. just kidding, but I need 5 really good classes, and some tactics. Whoever gives the most detail and whoevers tactic I get better with gets best answer. hanks