Running and gunning is definitely the best way to get used to the game and the maps. You should practice hip-firing whenever you are really close to an enemy- if you ADS the snap will go crazy.
uuum what else... don't be afraid to die! You'll learn the maps and traffic as you play more and you'll get better in no time.
Also, adding other people will vastly improve your game. You can check out their strategies and it's also a lot more fun! Sorry for the novel, good luck!