Right, well it seems you're spread right across the board then :/
I'll suggest playing soldier for a time (and also demo) and try to get at least one of them at a decent play level as those present the most FPS driven classes apart from scout/sniper. If you're into hitscan weapons (ones that do damage instantly where you're pointing when you click/no travel time) then I'd suggest you pick up scout/sniper. If you're into projectile weaponry I'd suggest soldier/demo. Very different classes, but relatively close in gameplay.
Are you euro based or North America based?
Here are some ammo mod servers. Join them and you'll understand quickly enough how they work. These should be used for training your soldier mostly, and possible demo/scout if you get a willing friend in there. However, the players in these servers are generally very skilled so I'd be cautious of going into this to begin with.
217.65.7.145:27015 - A russian ammo mod server ( I BELIEVE it's Russian :/)
217.112.254.24:27015 - An American based ammo mod server
67.202.71.165:27015 - The original ammo mod server (chicago based)
I'd also look in one of the introduction threads for a generic welcome post that has server info and vent info (I'm too lazy to go get it and my favorites list was recently wiped).
Also, what 3rd person games did you play (GoW?)
Lastly, I'd like to recommend that you go check out
www.quakelive.com for honing your FPS movement and aiming skills