If you're frustrated of getting your ass handed to you in Modern Warfare 3, you've hopefully come to the right place to get that turned around.
1.) Learn the Maps: This shouldn't come as a surprise, because it's really important in any game you play to be familiar with it. When I say learn the maps, I don't mean just play them a million times over going to the same area of every map (Which is why camping will not help you.), but explore. I have over 3 days played now, and I STILL see some people hopping up in areas of maps I didn't know existed.
2.) Beat the Campaign/Spec Ops Missions on Veteran: While the Call of Duties have gotten considerably easier with each coming release, if you can beat the game on veteran, then you should be in decent shape online.
3.) Do Spec Ops Survival: Sort of similar to number two, but even more helpful, as the maps you play on are the same ones that are online. When doing Survival Mode, you're both learning the maps, and getting a fair amount of experience with different guns and perks in the game. Not to mention it's actually pretty fun and challenging, too.
4.) Choose Perks Wisely: If you're playing a game mode like S&D, I strongly recommend you use a perk like Dead Silence, as it's the most useful for the game type (Although Sit Rep is also useful in S&D), and to not use something like Scavenger, as it's completely worthless in that mode. I highly recommend you make game-mode specific classes if you play more than one.
5.) Change it Up: If you find yourself playing one game type, such as Team Deathmatch, stop. Playing other modes encourages you to explore different areas of maps, learn choke points, and if you have a headset or are playing with friends, it encourages team work. That is, if you care about winning. But I'd assume you do, which is why you're reading this in the first place.
6.) QUIT CAMPING/Go for the Objective: If you happen to be one of many players in the Modern Warfare 3 community that likes to sit in corners during the length of game play, you really need to stop. What this does is two things: First of all, you only become familiar with one or a couple spots of any particular map. Perhaps more importantly, go for the objective. There's nothing more frustrating than being in a match of Domination and having somebody on your team playing like it's Team Deathmatch. I.E., in it for the kills.
7.) Pick up Black Ops: When in doubt and nothing seems to be working, get Call of Duty: Black Ops. Black Ops has the mode I talked about earlier - Combat Training - which lets you play multiplayer maps by yourself against bots at a difficulty of your choosing. It's really the best way to train your thumbs.