OCModShop published a complete walkthrough for Halo 3. If you do not have the time to figure out the missions yourself this is for you. Halo 3 launched last week.

Quote from the Halo 3 Guide: "You must follow your squad to meet up with Sgt. Johnson, and you don't have the luxury of an objective arrow (at the moment), but luckily your team knows the way. The Arbiter always has a blue arrow above his head, so you can easily find your bearings
Follow your teammates until you come to a waterfall. Once you reach the entrance into the open area, a Covenant Phantom drops the first enemies you encounter. Follow the path up to your right and fight the troops in the cement bunkers to your right."
Read the full Halo 3 Guide on OCModShop.