There is a lot to learn. For example:
-learn good crosshair placement. (aim at head level)
-learn the weapons (recoil, effectiveness, price effectiveness, etc.)
-learn map knowledge (know the map, callouts, nade spots, etc.)
-practice movement (big part of the game, you can watch videos about it and download maps meant for this)
-aim training (pretty self explanatory, just work on your overall raw aim)
I'm sure there is more I can talk about in farther detail, but I am not super knowledgeable at everything, though I would like to think I am very decent at the game.
Like I said earlier, the best thing you can do is find players that are very good at the game, and have them teach you some stuff. Learning from people who have great experience with the game is what you should do. I had a global teach me mirage one, and it was a wonderful learning experience. I still use a lot of the techniques he taught me and that happened over a year ago!
anyway, I hope this helps!