<< Headphones are a big help; they will allow you to determine where footsteps/shots are coming from.
Don't be afraid to "camp" (hide and wait for the enemy to walk by and shoot them from behind), especially when on defense. Often frowned upon on the servers where kids have just migrated from nintendo, but considered an art by the more mature crowd.
Watch the rest of the game after you are killed. You will quickly learn the major points of engagement, camping spots, etc.
The terrorist glock pistol is a bb-gun, buy a deagle in the first round.
Try to avoid these embarising situations if at all possible:
Killing yourself with a HE grenade.
Killing yourself by falling off a ladder, roof, etc.
Getting Knifed to death.
Getting Glocked.
Getting killed by kids under 7 years old.
Once you become addicted (it happens) you will probably want to write some scripts (macros) for buying equipment, sending messages, etc. with just one keystroke. Info is all over the web on this subject, just do a search when the time comes.
Good Luck, >>
I'd rather buy a USP than a Deagle if starting as a T.. You get 59 rounds instead of 21, 12 per clip as opposed to 7, and it shoots much quicker.. Of course, I've been basically using the USP with MP5/M4A1/AK everytime I play so I've got good at it..