Personally I print screen and paste to an external application when I'm done. Of course, sometimes I forget to paste it, which sucks.

Some days I have awesome games and melt faces. Other days I get steamrolled over and over. Just learn from your mistakes, find a weapon that you excel with, killstreaks that suit your playstyle and k/d, and keep your cool. A large percentage of my deaths come from getting pissed at some douchebag camping in a corner and going after him over and over again.
One thing you might do as well is watch some Youtube videos of folks playing - WoodysGamertag is a self-proclaimed "unexceptional gamer" who gives great advice for average joes like myself. Most of the vids are from Xbox, but the basic tactics are the same regardless of the console.
EDIT: XFreneticX is my Steam ID, add me if you'd like.