Crouch walking. If an enemy is near you crouch walk, you don't show up on the motion sensor then making is easy to sneak right up behind them.
Practice in FFA games & social playlists. Pick up a Battle Rifle at every opportunity. If you can't get good with a BR you will end up getting very little kills every game.
Combos! Shotgun Mauler/Melee for starters. It's a simple Shoot/punch as fast as you can at close/point blank range. You should always have ranged & close range weapons with you at all times. The Assault Rifle should be a last resort.
That's a little bit for now. If you need tips for anything else just ask.