Right where to begin

The learning curve in this game is rough, when you start you will get killed many times, and you'll probably have absolutely no idea where you got shot from. Many people will quit here, with less than 10 hours in the game, not knowing what could be. But don't let it get you down, push through the pain, spectate people that kill you, look at the kill cam and for the love of god be quite. Use all of it as a learning experience, use that dirty angle next time, flank people so hard that they wont expect it even if they had hours to think what you could be doing and slowly you'll start getting better and actually killing people. Then you'll start to see what i love so much about this game.
Once you get the hang of it, this is probably one of the most rewarding games i play. Managing to sneak around behind someone and catch them completely off guard, hitting a team of 3 campers with a flash and knifing all of them, or just throwing a frag grenade in and doing the same thing. Or you could be part of the Mosquito gang buzz buzz.