In Halo 4 they released a new targeting system - the one that is prominent in COD. The secret to this system is to stick to weapons that release an immediate shot - like the DMR and when you're zoomed in on the Lightrifle. The secret to these weapons is that the shot hits its target as soon as the trigger is pulled. Another thing to keep in mind is that they moved the kill box for people. Instead of shooting directly at peoples heads you want to shoot right above their head. Not too far above, but just enough.
My main advice to you would be to stick with someone. You are more likely to win a shoot-out if you're in a group. Also, be more of a defensive player. Instead of running directly into the battle hang more to the back and let the other players draw the people out. Stay away from the open areas. Hang to the edges and learn to crouch.
Also, Currencyy has some great advice but as he says he mainly plays COD. In Halo we don't really have many places that one person will tend to always go - unless they are a sniper. 343 makes their loadout drops random which changes the play of the battlefield constantly.
And if you're finding it hard to survive in the smaller team games switch to Big Team Slayer and stay in a vehicle. Hopefully that helps!