Teaching you how to aim better isn't something that can be done, and if you're doing bad, chances are that's the reason. No matter how much strategy you learn, how many unique tactics and skills you utilize, if you can't shoot an enemy there's nothing you can do.
And the only thing you can do to improve your shooting is just to play a bunch. DON'T use an AR, because that cheapskate weapon which isn't even good doesn't help you practice your aiming at all. I'd suggest start with the BR, then switch to the DMR/Light Rifle. Play Matchmaking a bunch, and you'll get better eventually.