1. Dont bother crouching, unless peaking around a corner into the next room, on most occasions, crouching just puts you head in an easier position for people to get a headshot

2. Get Flashbangs, while these are easily the most annoying and dissaponting part of the game, the proper use of these will get you alot more kills and make you look a lot more leet.

3. Go to a deathmatch server for practice. These servers tend to let you try and guns you want and with instant spawning will help upgrade your skillz alot quicker than normal servers.