I have CoD4 on 360 and PS3 now. And by the end of the week should have a decent headset for both. So online time!

I have not played any FPS online before. I assume there are several game types and learning the environments is key to them all (along with weapons that might suit each). But if you do play online what would you see as being good preparation in terms of playing the normal game? I would not expect to be excellent online by a long way but equally I don’t want to be a naive bullet magnet.
In the game so far I have got past the mission to protect the war pig (the mission before you get into the gunship). I’ve done up to there on the 360 and twice on the PS3 (first time was on my daughter’s profile so had to redo). That’s very limited experience I know and my ‘ohshititsagrenadethrowitbackquickBOOM’ isn’t so hot yet so I’d like to have more of a clue.
So where should I be?