Well the common thing beginners tend to do is they tend to take the shortest way to where the big firefights are. In doing so, you set yourself up as an easy target. You have to plan your routes out beforehand because three or more motherfuckers are doing the same thing.

Naturally you want to get that one kill right away, don't do that. Take routes that you notice aren't used by the other team. Stay out of the view until seconds before you unload a magazine on an unsuspecting enemy. Don't start shooting at a guy once you get a visual of him, make sure you're in good range of the gun you're using then unload. Otherwise you're going to be wasting your ammo by landing half your shots on him since you're not fully zeroed in, thus finding yourself to reload by the time he's aware which means him getting the upper hand.

Learn the other weapons, don't rely on your assault rifle 24/7. For example, if the enemy is far out I'll use a battle rifle, Plasma carbine, and of course sniper rifle. For midrange the spikers and assault rifle work well. For close quarters, use some type of shotgun, SMG or my favorite weapon of all..........the needler.