I love the game but have moved over to Gear Of War2 now.
Add ROMEO UK to your friends and ill play you, It also depends on what type of game mode you play.
p.s you do mean for the xbox 360 dont you.
I have played CoD 4 in the past alot too, when you're starting out in the game you're at a big disadvantage to alot of the players you're up against. As you gain points in multiplayer games you unlock new weapons, scopes and abilities.
As soon as you can start setting up custom classes with red-dot sites and get a feel for what weapons you're comfortable with. Additionally, alot of players play with extra damage and extra health perks called Stopping Power and Juggernaut. Once you unlock those you should find things quite a bit easier.
The good News
is that on your first run through (your first prestige) you accumulate these points pretty quickly. I'd recommend that you try playing in a fair few headquarters games until you get comfy as these award you heavily in points for team bonuses and also give you an option to hang back from the main action a bit more than the deathmatch maps.
The more you get to know the maps the better you'll get aswell.