I’ll echo quite a bit:
You have to know the levels; that should be a definite priority.
Stick with your team: if there are 2 or 3 of you firing at one opponent, the odds favor the group. Stick with someone, if you get shot at, the odds are that he will aid you.
Learn to pick your shots. If there is an enemy that doesn’t see you (has his back to you), take the time to aim for the head. Don’t take forever, though. I’ve seen the cockiest players take forever to get their aim right and then get shot when the guy turns around.
Use your sound. Get a good set of headphones and plug in. Footsteps are the one giveaway that an enemy lurks around the next corner. If you are the last man alive, always walk or crouch. This takes the enemy’s sound away, if he can’t hear you, he can’t prepare to shoot you until he sees you and you see him.
One you find the choke points in a level, use grenades to thin them out. Toss a grenade into the hallway at de_dust or lob a flashbang across the bridge on de_aztec. Use smoke grenades, too. This is a bit of a dirty trick, but if the enemies have high pings (latency on the TAB menu), ask your teammates to throw smoke nades into a choke point, this can drop the enemy’s FPS. It won’t make you any friends, but you might get a few more kills now and then.
Try to coordinate your team into sticking together. If you’re playing a 10 vs. 10 game, the 10 players together can almost always beat 4 or 5 guarding an area. This works very well if you are a Terrorist and are planting the bomb. (Rush across the bridge with your 10 and plant the bomb within 45 seconds of round start on de_aztec!)
Guns:
I like using the medium-range rifles (Steyr AUG and SG552). They combine power and accuracy, but don’t ever go full-auto with them.
Don’t ever use the AWP. Ever. If you do, you may as well type in the console: name “Coward” <enter>
The MP5 is pretty good, but it’s not powerful. I’ve put up to 8 MP5 rounds into one man and he walks away from the firefight. More expensive, but still good is the UMP .45. Everyone gives that gun shit, but it produces kills.
Pistols: The USP is really great, use it often. The Glock isn’t very good at all, put it on burst fire and aim for the torso, it will bounce into a headshot. The Deagle is marvelously fun, but you have to have the element of surprise. The P228 is a deceptively awesome pistol. It shoots a .357 round and fires at a nice high rate.
Use the machine gun often, it gives you more accidental headshots than anything else.
The shotguns are great, but, again, you must have the element of surprise.
Buy Kevlar each and every round.
Keep playing, it’s a great game.