The first thing you will want to do is make sure all dirt and dust are removed from the walls, from the floor to the ceiling. To do this, attach a cloth pillowcase onto the top of a broom with a rubber band. You can use that to remove any dust / dirt on the walls.
If there are any serious smudges, fingerprints or marks, you can try to remove them with a Mr. Clean magic eraser, which can be purchased at most grocery and drug stores, and definitely home improvement stores.
For the general wash, something mild (such as dish detergent) will work, use a sponge to apply and rinse it off.
Hope that helps!