try starting with the m16, gets you to Aim directly at your target to get kills, instead of just spraying..
Always move, yes camping sounds good with a decent place, but if no-one goes past said place then you're gunna get 0 kills arent ya?
stay away from juggernaut, Grenade launchers on guns, Martydom and Claymores... just makes you lazy when playing online, and aggravates everyone in the server with you.
Find a gun you can use once you can aim, this will mean you have something you're used to, thus making it so you can concentrate on playing rather than using the gun.
and 1 simple one... Get good with a pistol, without it you'll spend alot of time dead.