What happens next?

How to win at halo 2
Learn the weapons, starting with the battle rifle.
Halo has a lot of weapons, and the more you know about them better you will be in any mode. The essential weapon to learn is the battle rifle, which shoots accurate , 3-bullet bursts suitable for a variety of distances.
With proper shot placement (the head), it can take down most enemies in a few hits, and the scope greatly increases your range. The covenant carbine is a similar weapon, though slightly less common.