Okay, here's how to be good at COD.
Firstly, DON'T TRY TO DO ANYTHING IMPRESSIVE. If you can't no scope, quick scope, whatever, don't do it until you've had practice in a private match with friends!
Second, always have a class you are comfortable with. I'd recommend using stopping power with an assault rifle, preferably one with little recoil. Unless you know how to snipe, I'd avoid having that as a class as well. Assault rifles will be your best friend in many scenarios.
Third, don't always sprint, and know when to hold back. I can't tell you the number of times I've seen people blindly run around a corner just to get killed by a hail of bullets. And if your team is being badly dominated, hold back in friendly territory. Just try and defend that spot at all costs, and once a break appears, try and get some people to make a move with you.
Fourth, camping is a legitimate tactic. It will make many enemies, but you certainly should die a lot less. But what you don't want to do is stay in one spot for too long. Move to a new location, get a kill or two, move somewhere else.
Fifth, always have a UAV as a killstreak. I have joined so many teams where no one else has a UAV, and we're left in the dark for a majority of the game. It only takes 3 kills, and it helps everyone on your team. But this doesn't mean rely solely on it; it takes a second to update, just enough time for the person you were watching to run around a corner and kill you, and there are cold-blooded people.
Lastly, and most importantly, REMEMBER THAT IT'S JUST A GAME. Many people get frustrated over this game, myself included. The easiest way to avoid this is to just relax, grab some buddies, and goof around in the game. If you want to be more serious though, there's no shame in leaving a server where you're being badly dominated. Just find another match, maybe a different gametype, and start over. Or if worst comes to worst, just turn off the game for a few minutes, take a deep breath, then do something else for a bit.
