After exiting your Warthog, jump across the bridge and kill the Brute and two Grunts attacking your men. Turn directly around, facing toward where you came from, and look to your left to see a structure. Go up to the wall on the structure and look to your right to find a pathway. Follow the path until you get on top of a bridge-like structure. There should be some dead UNSC Marines, a sniper rifle, and some other ammunition.