1) Start playing single player
2) Finish single player on normal difficulty
3) Get good enough to beat single player on hardest difficulty
4) Start playing multi-player (and this is where you deviate from the expert plan)
5) get good enough to beat people online
You'll very probably never be the best, but all you need to do is to be able to hold your own against the average player. At Christmas, I was truly hopeless at TF2. I kept playing with the heavy, and soon those 1:5 k/d's started to get more like 1:2's, then finally 1:1's. Now? Now I think to myself that if I'm not at the top (unless there's a VERY good reason for me not being at the top- as in I'm being outplayed by 3+ players), I'm not playing well. Some games I'm managing closer to 5:1 over the whole game, but somewhere between 1:1 and 2:1 is about right.
tl;dr- Just keep playing multiplayer, eventually you'll learn the tricks that mean you die at the moment, and then you can use those tricks yourself.