Brian and Kuros give a lot of really good tips to start off with and become better with, as well as Falcon amazingly. I’d have to completely agree with the both of their points but to change up and add a few things as well.
The P228 is a great pistol, great for rushing and pretty good when not. If you’re going to go for a long-ranged attack situation (awp, scout, etc) get a DEagle. If you’re not going to rush, get a DEagle. The DEagle has a better accuracy when firing from long distances, its damage is more potent and has a better overall effect from a distance. Up close the DEagle lacks the ammo in clip and the firing rate.
As much as you probably don’t want to, pick up an AWP or a Scout. AWPing and Scouting is possibly one of the best things to do to quicken your reaction times. The faster you can react to an enemy crossing your crosshair, the faster you can react to them with an AK/Colt. Not only will AWPing and Scouting help you overall with your reaction time, it will help with knowing the wherabouts if your team as well as the enemy. If you sit some place with the AWP while your team goes another, more than likely you’ll get flanked and killed easily. Depend on your sound, through and through. Sound will save your life countless amounts of time, whether you hear people coming or whether you hear gunfire in another direction.
Throw flash bangs and smokes. I’m terrible at this sometimes myself, but learn to throw flash bangs and smokes -all- the time. You’ll learn that 9 times out of 10 when you throw a flash, someone is going to be flashed. If you think the enemy is there, throw it. Them being blind is only an advantage to you.
As Kuros said, use bursts. Far distances use a burst with the AK/Colt. Typically a 3-4 round burst starting at the chest of an enemy will end up going straight to the head. Go play on some aim_ak-colt maps and learn to hit from a distance. The quicker you learn this the more often it’ll save your life.
Quicken your R/T, learn the maps in and out, depend on sound, and use burts. It’ll help you out in the long run.
Once you feel good enough, I suggest getting on IRC and on Gamesurge, go to #findsourceringer and play some Scrims with some clans. Scrimming and playing in matches will increase your skill 10-fold. Learning to play with a team and not bait your team is crucial to the survival of yourself, and your team.