The same way you get better at anything - practice. Some consistent and intelligent practicing will get you playing better than 90% of people in a very short amount of time.
Most people develop their skills over time by just playing the game. If you seriously want to get better, though, you need to take it seriously. You can improve much much faster than the normal person who "just plays" just by doing a couple simple things every day.
Figure out what the "fundamentals" of the game you're playing are, and lay out a practice routine based on that and do it every single day without fail.
For example, when I was seriously playing CS:S in competitions, this was my routine:
Playing against 3 weaponless bots, get 100 kills with the glock, and then switch to the usp and get 100 kills. I did this because I found that one of the weakest aspects of my game was the pistol round - so I designed exercises to improve that aspect of my play. Which leads into....
30 mins of deathmatch server using only glock, and 30 mins of only usp, and finally 30 minutes with the deagle (my favorite
100 rounds of long range shooting with the ak/m4 (switching weapon after 50) - I would get a teammate and we'd go find a long corridor and just strafe back and forth shooting each other.
100 rounds of close range shooting - same as above but close range
Lastly I would work on my Awping - I would use a scout in a deathmatch server for 100 kills, and then AWP for 50.
After that I would usually scrim or play 1v1's with people who were better than me.
Also, whenever I wasn't scrimming or in a match I would have my crosshair turned off - I needed to break the habit of always looking at where my crosshair was and relying on it to aim too much. You need to be paying most attention to the corners of the screen, that's where most things happen - don't be focused on the little lines in the middle. You should already know where they are (they never move)!
I obviously had way too much time on my hands, and was able to put in obscene hours to get better at the game. It doesn't take a large amount of time if you just want to be better though - figure out how much time you have to devote to the game, and then lay out a routine based on that. You will improve more in 10 minutes a day of serious, focused practicing than you would in hours and hours of regular play.
Just make sure you play every day (even if only for 10 minutes), and you are focused while doing it, and you'll see results very quickly. Keep in mind that growth is painful - it will be frustrating, mentally tiring and you will want to stop and just play. That's a good thing - if you aren't struggling with it, you aren't working hard enough!
Oh, when I first started out playing CS:S I didn't get a kill for like a week. I had terrible terrible aim - here's a little exercise I made up that really helped me:
I would go into a map all by myself and just target random things - swinging my crosshairs from one corner of a box, to the other corner, back, to the flower pot beside that, etc. Just practice moving your crosshairs around and targeting specific things - just doing this for a solid 10 minute period every day should really help.
Oh, I forgot one more thing! I used to do a reflex tester thing every day for 15 minutes to get my reaction time up. You probably don't need to worry about this, but if you're interested I like this one - http://www.happyhub.com/network/reflex/
I would also highly recommend watching videos of the pros playing - you can really learn a lot by just watching how they move, react to sounds around them, etc. Don't watch the ones on youtube, go here and download demos of people who actually know what they're doing and aren't just trying to impress people with cool kills - http://www.gotfrag.com/tf2/demos/
Oh, and one MORE thing - don't worry about people moaning about your noobishness. Everybody starts off as a noob, and 99% of people who play any game stay that way
. Even most people who play competitively are pretty bad, and no matter how good you are there's always somebody who could own you without batting an eye. So don't ever talk smack, and pay attention and learn from guys who are better than you. If somebody is really pwning face go spectator and watch them, or just follow them around and hope some of their skill rubs off.
Don't pay attention to the guys whining about having you on their team - they're probably awful too, and you'll be better than them in a week whereas they will never really improve.
Whenever you die, try to figure out where you went wrong. What killed you, how did you get into that situation in the first place? Why did you lose that encounter? What should you do differently next time? How can you avoid letting that happen again?
Always be analyzing your playing and thinking about what you could do differently to improve. One handy tactic for this is to record yourself playing, and then watch it later. It's usually very easy to see what you did wrong (and what you SHOULD have done) when you're not wrapped up in the heat of the battle and are watching as an objective observer.Edited by pelirrojo - 9/9/08 at 3:54pm