Jesus.. what do you guys do to get such high kills?
Practice. I'm sure I've dumped over 1,000 hours of my life into the COD series alone. Maybe 2,000. I'm not sure I want to know

And that doesn't count all my multiplayer FPS training which dates back literally to Doom.
I'm ok when i break the double digits but i usually am the guy who gets killed the most in any map
In COD:UO back in the day, my goal was to be at the top of the kills list every game, and if that meant being tops in deaths, so be it
It helped a bit when i started to move more carefully but i still get shot in the back, hit by f...g campers and the occasional "real" sniper (as opposed to a camper) like there's no tommorrow.
Currently i'm trying SMGs and it has improved my k/d ratio a bit.. i'm not the most precise shot (i've seen guys who hit me the next millisecond they saw me.. amazing) so i'm wondering what are some basic tactics and weapons to improve my game (i know it's a very broad subject but still).
I feel like I could write a book on this. Maybe an incoherent mess of a book, but a book nonetheless...
Don't play like a bitch - That is, don't camp in the corner waiting for some poor bastard to run by. You'll never get any better playing that way. It sounds like you have that covered, but it bears repeating. If you go a full round of Team Deathmatch and end up like 4-1... despite the 4.00 KDR, you blow. You are useless, and you were mostly a non-factor in the mach. Don't be that guy.
Gear - Have good gear. This is maybe the easiest way to improve, well, if you have the will and the disposable income to improve your gear. I'm not sure if you're playing on console or PC, but either way the best way to go is a nice 1080p screen with low input lag (maybe even higher resolution on PC). I have an Asus VW246H which is pretty great for Xbox COD. And also a Dell U2412M. Turn the in game brightness up a bit. Beyond what the little configuration says... where it says "NOT VISIBLE" you want it to be decently visible. Not so the game looks annoyingly washed out... but you want it enough to where you can see enemies better in shadows. Also make sure your console is outputting 1080p, or whatever the native resolution of your screen is. You don't want your TV or monitor rescaling the game for you... let your PC or console handle it...trust me. A proper monitor will let you see enemies you would otherwise miss, and possibly shave 50ms off of your reaction time. That's pretty good for not having to make any actual improvements to your gameplay!
I also recommend headphones. For consoles there are even MW3 branded Turtle Beach brand headphones that should be pretty decent. I've done really well with an Astro Mixamp and some high-end Sennheiser headphones, but they sell an A40 package which is probably more convenient and better suited to games. Headphones of some kind are very important. Or maybe if you have a really nice 5.1 system that you can crank up which is also properly positioned and calibrated. If you can't tell exactly what direction peoples' shots are coming from by sound alone, you're doing it wrong. With SitRep Pro you can tell from footsteps alone exactly where enemy players are. Unless they are running Ninja. But very few do in Core gametypes. In public Team Deathmatch lobbies this is probably biggest advantage you can get over other players.
Ok, well, maybe not the biggest. I also run a Xim3 keyboard+mouse -> Xbox adapter. It's amazing for COD. Expensive, and a bit of work to set up properly, but it's basically as accurate as my computer mouse. I really didn't think it was possible. Failing that, maybe some FPS Freakz or whatever they're called. They're little thumbstick extenders that give you better accuracy in manipulating those contemptible little console joysticks. On the PC end don't underestimate a quality gaming mouse and pad. I'm a huge fan of the Mionix Naos 5000 on a Steelseries HD hard mouse pad. The thing moves like a hockey puck on fresh ice.
For controllers, you're not going to want to use the default sensitivity settings... You're going to want to crank them up a bit. It will take some getting used to, but you need to be able to turn faster than the default settings let you. If you are on Xbox defaults I suggest going to maybe 5/10, and sticking with it a while. You will be awful at first, but in the long run you'll probably be better. On the PC end crank your gaming mouse DPI as high as it'll go, and then adjust the mouse slider in game to get the preferred sensitivity. That should give you the best accuracy.
Learn the maps - And I mean, really learn them. You need to know what angles you are potentially going to be shot from at any given moment, and minimize them. You need to know where every bit of cover is, every cheap sniper spot. You need to know what your flanking options are for the guy you see right in front of you. You need to know if you're close enough to run to overhead cover when you see an enemy Predator missile pop up on your minimap. Also, at the start of a round it's nice to have an idea of when the first enemies will pop as you're rushing. Getting that first kill and breaking through their front line leads to nice flanking opportunities. If you understand the timing of the map well you will consistently put your team ahead early.
Watch the Killcam - I can't stress that enough. This is your greatest learning tool. It'll help you learn the maps when you see some guy shooting you from a piece of cover you never noticed. Hell, just today, and keep in mind I'm closing in on 6 full days of in game time for MW3, I learned that you can shoot down through a certain window from the catwalk on Carbon. I had no idea.
But the killcam gives you a wealth of knowledge. You'll see the enemy's weapon and perk setup. Copy the good players. Maybe they have a combo you never considered that will work for you. Most importantly... figure out how you died! Sometimes it's lag, sometimes you just got out-aimed, but other times you can see mental mistakes you made which you can correct. Maybe you overstayed your welcome and your shots gave away your position making you incredibly easy to flank. Maybe he had superior cover. Maybe he had a better weapon for engaging at whatever distance you engaged at. Maybe you just left yourself exposed from too many sides.
Stay away from your teammates - Your teammates are idiots. Trust me. Expect them to do whatever idiotic thing is possible to get you killed at any given moment. My advice is to stay away from them as much as you can. If you have been watching the killcams, you will see how many times your idiot teammates get you killed through inaction or simply getting in your way. Running Blind Eye/Assassin/Ninja with a silencer? Too bad, because the moron two feet away from your made you collateral damage via a Predator, or he shot at absolutely nothing and put a nice red dot on everyone's map right where you are.
Keep him at a healthy distance, and his stupidity might even net you a few avenger kills. Teamwork doesn't really exist in pub lobbies. Don't think that you're magically going to make it work. The only one you can depend on is yourself.
Control the Engagement - If you are running an SMG spray n pray class, take tight corners, stay away from long sight lines, don't put yourself in a disadvantage. If you're running an Assault Rifle, go wider, use long sight lines, Aim Down the Sight aggressively, use cover, don't make it easy for some retard with FMG9 Akimbos to make you explode from two feet away. After you get a few kills, relocate, you have angry people respawning that know exactly where you are. And since this is MW3 they will spawn in completely random unpredictable places. You may only have two seconds to avert imminent disaster.
There is No Shame in Retreating - Fall back a few feet to reload. If you shoot a guy once or twice, and he shoots you once or twice, and you're one or two kills from a killstreak or something, there is no law which says you must take the 50/50 chance and finish the battle. Retreat. Flank. Run to the opposite side of the map behind all your teammates. You should be learning how to get the drop on people... at a certain point in your development as a COD player, a "fair fight" is really putting yourself at a disadvantage. Why play "next bullet wins" when you know how to flank him? When you know a more advantageous situation that you can flee to? No need to be a hero.
Learn the Game Mechanics in Depth - Google "Hey Den" and read that message board for a bit. Look at the recoil charts. The REAL weapon stats (the stupid sliders inside the game lie. They ALWAYS lie). There are worthless weapons in this game. There is no reason to use an SMG that isn't the MP7, the P90, or the PP90M1. Understand that an AR class gets more benefit from Stalker and Quickdraw than an SMG does because an SMG already moves quickly in ADS and has a faster ADS delay. Attachments don't always work the same on every gun. Did you know that Rapid Fire increases fire rate by over 30% on a PP90M1, but on the MP7 it's only some 18%? Do you know the difference between how the Kick proficiency and the Stability proficiency work? What idle sway is? How fast various weapons reload compared to others in their class? Do you know how to 'reload cancel' to finish your reload more quickly? You should! And these are not things that are in the instructions or in-game menus.
Don't waste anything in your custom classes - Make sure you have a good reason for every perk and attachment you use. Make combos that work together. On ARs I think Kick is the best proficiency by far, and in most cases I would put a silencer on top of that. On a Type 95 it would be rapid-fire. It's always tempting to put on a nice red dot sight, but as a red dot sight junkie I still say it's in your interest to learn to deal with iron sights. The silencer does keep you alive. I'd stay away from LMGs, Shotguns, and Sniper rifles altogether. LMGs really have nothing going for them. Massive ammo capacity I guess, but you move slow as shit, you generally have much more recoil, you don't do as much damage, and bullet penetration isn't as useful in MW3 as in previous MW games as best as I can tell. The maps are too small for sniping, and shotgun range is too limited, they're not even remotely useful until you spend a few hours leveling them to get the good proficiencies and attachments. Play those classes for fun, but don't expect to put up monster scores with them.
I guess my final random thoughts on class creation are as follows: Keep a Stinger class around to keep your team from getting dominated by attack helicopters. Suicide to get to it if you have to. Don't bother with pistol secondaries. Go FMG9s or MP9s akimbo if you want something for close range, and go MP9 with a red dot sight or silencer if you want to be able to handle medium range with your secondary in a pinch. Extreme Conditioning is a pretty useless perk. Overkill is a pretty useless perk. Marksman is ok, but only situationally. AR, larger map sight lines, and against teams without a lot of Assassin users. Ninja is really only useful in FFA and like S&D. In most of the other game types your footsteps just aren't going to give you away. Even though SitRep Pro is awesome, and I suggest you abuse it, the fact is very few people do use it. So Ninja only counters a pretty rare type of player. You want perks that help you against average idiots, because average idiots make up 90+ percent of the game population.