I agree, the M4 is a great weapon to start with and Stopping Power will help you kill with as few bullets as possible, while you get aiming under control.
Ninja is also a great perk that I run on most of my classes, since I don't find many of the other Tier 3 perks as valuable.
I might also suggest turning up the sensitivity on your controller a couple notches. It will seem REALLY weird at first and you will suck, but when you get used to it you will have quicker aiming, "like everyone else."
Also, I would suggest not playing during peak times (evenings & late night). All the uber-geeks are on then and it can get downright frustrating.
MW2 takes practice to be good, I won't argue. Otherwise it is just irritating.
Also - if you have access to the TAR21, give it a try. It is not terribly accurate at a distance, but has good stopping power and fire rate at close range. It's even better with stopping power. I would say it trades accuracy for power, compared to the M4. I like it a lot for when I run & gun, which is most of what I do.