This game has a sort of "old" community that lingers. You'll know what I mean when you start playing, but that isn't all bad. There is usually a single big server (15-20 players) and usually 1-3 small games (4-6 players).

First thing you need to know is the classes.
2 can turn invisible for some time, each one has a special vision type.
C to go invisible.
V to turn on vision mode.

Recon- Runs faster, least health, can turn invisible, night vision
Assault- Walks moderately, medium health, can turn invisible, movement vision
Support- Moves like a turtle, can get beaten with a brick and act fine, as visible as a road flare, heat vision so you see everyone all the time.

Why use recon?
To get on to roofs and high places and hide.(C4 CALLED DETAPACKS)

Why use Assault?
Because you can see people who move, even when invisible. Can take a good hit too.

Why use Support?
You basically can get shot in the face with a shotgun, twice, and be fine. Not only that, you'll know they were there for 10 minutes before they attacked,

Maybe I exagerated a little bit about support, but it's pretty close.

You pick your class, you pick your character model, and then you're in.
Each game you play in several matches, once you die, that is it for the match.
To win, you must take the ghost (the flag) and deliver it to the green arrow, before the enemies deliver it to the red arrow.

Or just kill all the enemies, that works.

Anyone holding the ghost has roughly 10 seconds, and then can see all the enemies through walls (or see how far they are and in which directions.)

Learn to work with your team to capture the ghost and deilver it, or just stomp the enemy and kill them all.

Watch your team, do as they do, learn some tricks from the good players.
Learn what not to do from the people who die in the first 20 seconds.

Go assault primarily, you get well rounded stats, and invisibility.

Don't rush recon, just don't. It seems like a good idea, but you'll get hit with a supa in 10 seconds.

Support is not a god, don't treat it like some kind of immortal monster, just kill it before it kills you.

Line your cursor with the edge of a wall when you turn, and slide the cursor to the next corner, so you don't have to move as much to get a good shot.

If you see someone invisible, walking away from you, don't take the shot unless you can make it. Good players WILL turn on you and WILL make you regret the shaky shot. Just catch up to them.

If you play recon, hide and get behind the enemy, outsmart them and you can kill them.

Have fun.

Play the game if you want to learn, because MOST of your experiance will be gotten through playing, not reading guides.

Good luck, and have fun.