go to the left slightly and there is a little walkway down to the ground level, take your warthog down there and to the beach. then go to the left and even tually you will reach a large rampish/rock thing. drive it over that and into the group of covenant . Then kill all of them and walk over to the overturned warthog by the beach ,grab ammo, health as needed and the overshield. then head back into the trees and you will find a path that leads into the center of the island. after that it's pretty much self explanitory.
ware say you got get out the buliding. I don't know what do after that