OK HERE IS THE GUIDE FOR A NOOB IN CS OR CS:S
RIFLES:
Best for CT is either the M16 (or M4 i forget) or the AUG
Best for T is Defender or AK-47
SNIPER RIFLES:
AWP or Scout, scout tends to be more accurate at longer distances, but awp is most effective for easy kills (note: once you learn the art of quick switching [aim your AWP, fire, press "q" twice] it kinda helps you to view you enemies better) and if you have a VERY responsive mouse, you can become a very good sniper.
As for semi-auto snipers, they are good for close-medium range kills (crouch for best effect, 2 shots to the upper-chest area should do the trick if you are accurate) also, if you are have zoomed in already, WALKING (not running, but hold shift to walk in CS) while aiming at a certain area also helps, sometime if you are suddenly confronted by an enemy.
ALWAYS REMEMBER WITH RIFLES: THEY ARE ABLE TO PENETRATE WALLS, so if you are suspecting an enemy hiding behind a box, shoot at it a couple of times, you might hit something!
Other guns:
Deagle (only if you have a decent aim, in my case, not very good aim, so i dont use it much) is effective.
Pump Shotgun (B-2-2 is the buy hotkeys) can help, and in some ways better than the auto-shotgun, it limits your ammo, so you dont waste all of it at once, and i think its slightly more powerful to the auto.
Flashbang: after you have been flashed in CS:S dont expect to live very long, unless you have a quick way out the situation. i find that the flash impact is much more devastating in CS:S than in CS, and people actually use it much more in CS:S rather than CS.
Frag Grenade: the one thing i hate about them is that they never to enough damage, but if you are lucky that some teammate has damaged an enemy, then you can try and kill the damaged enemy with your grenade rather than risk rushing at then instead to be killed rather then to kill.
Vest & Helmet: always try and but a helmet if you have the money, if not at least have a vest, if not that have a primary weapon (a decent rifle or SMG will do, even with out a vest). i would much rather by a rifle with a bit of ammo with no vest rather than a vest with a crap handgun with tons of useless ammo (due to the fact that if you auto-buy with little money, it will get the worst selection for you, but you never know, it may work out for you, other wise, if your short on money, save it for a next round or buy a decent gun that you know you can kill 2 or 3 people with.
hope that helped for now (thats just basics....)