The hottest water you can stand, a sponge, and a putty knife. Get the paper as wet as you can so that the glue will release and start scraping. You can try one of the gizmos that scores the paper, but I think they make more of a mess than they're worth. Put down an old sheet of plastic on the floor that everything drops onto and you can just wrap it up and toss it into the trash after. Makes cleanup easy.
You may need to pick up a pole sander for any remaining adhesive once everything dries. A handle is probably 4 bucks, the mount is probably only a couple, and you can get a pack of fine screens for under $5. Sand the wall off to a smooth finish and start priming. Ask your hardware store for a good primer / sealer to use based on what type of wallboard you have.