Some of it IS lag; having a 3 bar connection in any CoD vs somebody with a 4 bar connection puts you at a slight disadvantage, but it's still just enough to make you die half of the time, even if you aim and shoot first in a 1v1.
MW2 was an EZ mode game; you have to put it behind you and try to forget that game as much as you can, especially if it was your first CoD. None of the guns had recoil, and the killstreaks were ridiculous. One thing I would suggest is firing your weapon in short, controlled bursts of 2-4 rounds, unless you're at close range, then you don't want to stop shooting until they die.
CoD strategy is a mix between camping, moving, and rushing; you just have to learn when to do what, at what time, and you'll figure it out eventually through experience (don't feel too down, we all start that way). My advice is, if you're camping, get a few kills, then move, unless you like explosives crammed up your ass so far that they begin to fall out your mouth (which I'm sure you don't, lol), because people, quite frankly, don't like it when people camp. Don't constantly rush, but rush people when they are unsuspecting, they have no Spy Plane (or you have Ghost), or you know you can beat them from raw skill/connection. It's all about timing, and where you look, at what times.
The Galil and FAMAS are good beginner weapons. Don't use the M16, M14, or the shotguns until you feel more comfortable using other things. If you like to rush more, use an SMG; you pull up your weapon faster, have a better ADS speed, better hipfire, and a slightly faster movement speed than players who are using assault rifles. If you both are at close range and ADS, if you simply shoot them while walking to the left or right, chances are that if you're close enough, you'll kill them easily. The only thing is, a lot of people hipfire, and with Steady Aim, they can prove to be troublesome. Your SMG still has better hipfire and mobility though, so don't stop moving when you shoot them. I also recommend a suppressor on SMGs, because you should be using them from close range anyways, and it shouldn't really affect the damage at the range you'll be using it.
Use lower killstreaks at first. Don't be the guy that uses 8,9,11, and never gets a killstreak past 3 for 5 games straight. You'll be helping your team and yourself more by picking low killstreaks, and you'll be having a lot more fun on top of it.
Go into Combat Training to get the feel of the game; rank up as much as you can, get new guns, and pick which ones that you think feel the best. The FAMAS and AUG have slight advantages over the other ARs at close range, but lose their damage slightly faster through range and walls, along with having a high RoF and goofy recoil. Every weapon feels as if it has a purpose in the game; the AK, for example, shoots slower than a lot of guns, but is VERY stable to aim and shoot with, and I personally love it, because I'll never be screwed over by the recoil randomly jumping in some direction, which happens ever so often with a lot of guns. The only weapon I say to stay away from is the Olympia; that thing is a POS.
Good luck.
