Tips
Buying the right guns is something you should learn after the basics. Rifles (not Snipers) are the most accurate. Pistols will save money but don't expect to be a hero with it
Aim for the heads without crouching. Crouching gives your enemy a still target. Aiming for the heads will almost always give you a one-shot-kill.
Know the map. Not knowing where the approach points are and the objective point (bomb site, hostage site, escape site) will really hurt you.
If over time you get good enough look into joining a league like CAL (the CyberAthlete Amateur League). These leagues can be fun and you can earn respect and in some cases money.
Buy armor and a helmet, it will greatly increase your chance of survival.
Warnings
Don't Hack. This game is hard to master. Don't revert to hacks, for the greatest moments in CS are when you kill someone by yourself, not with hacks. Not to mention people hate you and can usually tell if you are hacking. If an administrator catches you hacking, he can ban you.
Respect the administrator of the server. He controls whether you play or not.
Be mature. The most annoying thing in CS is an immature 10 year old shouting "Hacks!" when he gets killed.
Headshot is always a good thing. You could kill another player with one shot to the head!