This is just a simple list of tips and techniques that can help you improve at Call of Duty and most other First Person Shooters. If you disagree with anything feel free to reply as I want this to be as accurate as possible. The list order is pretty much arbitrary. Here we go.
1. Don't beat up yourself. This means don't assume you're hot **** unless you are actually the top clan and even then don't let it go to your head, that's when you start to slip. That means don't make excuses like "Oh ****, lucky shot noob" or "noob spammer, use a rifle!!11". Instead of making excuses try to see what you did wrong and then improve on that. You got sniped, maybe you should have naded first and then peaked so the sniper would have to move rather than nading and then waiting 15 seconds.
2. Be confident. This isn't contradictory to number one. Confident means don't belittle yourself or your team. If you go into a scrim or match thinking you're going to lose then you're already at a big disadvantage.
3. Find your right sensitivity. This is pretty simple to do, simply find a small object like a lantern or even shoot a hole in a wall. Then strafe left and right while trying to keep your crosshair on the target. Adjust your sensitivity until you can do it or come very close. Sensitivity is the part of the basis of good aim.
4.Practice, practice, practice. Try to scrim or pug as much as possible but not to the point where it becomes a chore. If you don't enjoy it then chances are you won't learn anything. Practice as a team too, jump in an empty server with your whole clan and go over everybody's position in detail. This includes good nades.
5. Demos are the key. Watch a demo but instead of watching for cool kills analyze everything the player is doing. Why did he throw that nade? Did he expect a rush? Asking 'why' will greatly improve your skill level assuming you can figure out why the person actually did what they did.
6. Aim isn't everything. Just as important as aim is the ability to outsmart your opponent. No matter how good of a shot he is he can't hit you if you get him from behind. Always try to stay a step ahead of your enemy, this means predicting what he will do. If get into a firefight with somebody who has a sniper in close quarters you can expect him to run behind a wall and pull out a pistol so adjust to that. I can't stress how incredibly important 'thinking' and knowing what the enemy will do is.
7. Communicate. If you see an enemy call it on the mic straight away. Communication is the difference between a decent clan and a good one. Getting naded like hell at A? Call 'Lots of nades at A' rather than 'Rushing A!'. Be sure of your calls before you make them.
8. Be a good team player, that means no calling other teammates **** if they die. Generally just be a nice and polite person in general. Of course you can joke around and talk **** but just don't start going over the top.
9. Moral. Team moral is something that can be hard to control but you really have to put in the effort and try to. That means no raging on the mic when you die (or ingame for that matter). No whining or crying either. If you die just say 'Two PPSHers rushing A'. If you're playing a match and you win a round try to be enthusiastic and tell the others that they're doing a good job. Don't make fake compliments, that doesn't help.
10. Momentum. Momentum is huge, if you win 5 rounds in a row the other teams moral can be pretty much shot. Say things like "Great work guys! Keep it up, we got this round."
11.If you're a pubber (i.e somebody who doesn't really play for a clan) and you want to get good try joining a clan and see if you enjoy playing more seriously. If not that's fine, competition isn't for everybody. Different strokes for different fokes
12. Very important thing to note: Everybody has the same potential. That means you can be just as good as anybody else, all it takes is some practice. Different people improve at different speeds so don't be discouraged if you don't become a lot better in a week, it's a process.
13. Strategies. Strategies are as essential as aim and thinking. If you don't have somebody in your clan who makes strategies then find somebody or better yet, do it yourself. Watch demos and see where each player goes, then ask yourself things like "Why did he send two to B and the rest to A instead of all to A?", "Why did they fake B this round?". It's not stealing strateiges, it's looking at what others are doing and learning from it.
14. Sportsmanship. If the other team starts complaining about something that isn't a person lagging out or something serious like that ignore them. Always say gg (good game) after a scrim or match, not because you have to but because it's a sign of respect and courtesy. Don't complain about what your enemy is doing as long as it's not hacking or glitching. If they keep rushing with LMGs then adjust to that rather than calling them noobs.
15. This is something I forgot to mention. It's similar to number 5 but it's pretty important so I'm giving it it's own number. Watch your own demos. Try to see what you did wrong and why you died. Did you peak when you shouldn't have? If the answer is yes then try to work out why you shouldn't have peaked, maybe they had a sniper at the back, maybe you had a rifle and it was a PPSH rush. After you see what went wrong and why it went wrong work out what you should have done. Instead of peaking maybe you should naded first to make them hide and then picked when they peaked again.