1 - Experiment with guns, classes, killstreaks etc. You need to find out which guns work best for YOU. Everyone plays differently and your weapons need to compliment your style. For me, I use either the Famas, Galil, AK47 or Commando, each with a silencer. Secondary weapon is either a Crossbow, a Strella (I think it's called) to take down helicopters and stuff, or a Magnum with speed reload (I think it's called a Python). Either Frags or Semtex, Flashbangs and Claymores. Perks - either Lightweight Pro, Scavenger Pro or Ghost Pro, but mainly Ghost Pro. Always have Sleight of Hand (for me this is a necessity) and finally either Ninja Pro or Second Chance Pro.
I like to run-and-gun, but also like to keep myself off the radar, that's why I mainly use Ghost Pro. Sleight of Hand is very useful, although once you use it too much you tend to depend on it, which is why some people don't use it. And Second Chance Pro is good because you can get revived by teammates, but at times they don't revive you for whatever reason that may be.
2 - Stick to the edges of the map. No need to go running in to the middle. You'll get picked off every time. But sticking to the edges means that there's one less direction from which you can be killed, so it helps out a lot.
3 - Play Combat Training. This is a great way to familiarise yourself with the maps, and the scores don't matter to your online combat record, so even if you do terribly bad it makes no difference. And knowing the maps better means that you get a better idea of where the enemies are hiding.
4 - Always check behind you. Almost every time I move past a corner I check behind if anyone's there. This may be a bit tricky at first but after a while you'll get the hang of it.
5 - Be a team player. Unless you're playing Free-For-All, you're part of a team, and you need to work together. If you're playing something like Team Deathmatch or Domination but all you care about is yourself, then it's best to go to Free-For-All. It's so frustrating for everyone when you lose a game but a guy on your team has about 60 kills and 4 deaths. That's frustrating behaviour. Play the objective if it's required.
6 - Try not to rage quit. If you get annoyed at the game or other players, try to keep calm because when you're angry you tend to do stupid stuff which gets you killed and in turn feeds your growing agitation. Everyone gets annoyed playing these games, but raging doesn't help. And if it gets really bad don't leave the game, because that could mess up the connection and the lobby might close, thus ruining that game for everyone.
7 - Let time do its job. The only guaranteed method to getting better is by playing the game. The more you play the better you'll become.
When I first got Modern Warfare 2, the first Call of Duty game I've ever played, I was horribly bad online. My k/d was less than 0.5, and to be honest I got a bit annoyed and stopped playing for a while, although this was partially as I had exams that I needed to study for. But when the summer came I got back on it and did the things I mentioned above, apart from inapplicable ones such as Combat Training, and they helped me so much. By the time I got to Black Ops, my k'd in MW2 was above 2.
So if you listen to everything I've said, and no doubt other things that other people have said but I've missed out, then you'll easily improve at Black Ops. Check Our Call Of Duty Morden Warfare 3 News Every Friday