Tips for low ranks?

Pick a flash character in the lower ranks no one will ever turn away from a flash and you get easy kills.
On a more "serious" note, if you are new to FPS, try aim trainers. Movement? Watch Youtube about counter-strafing and how to peek certain angles. Game sense? Comes with playtime and watching pro vods.

Once you get better, aim becomes less important and you need to get a brain.