How do I repair a hole in drywall, and what do I have to put inside the wall?

If you put spackle or mud in the hole, you cannot use it again with just a screw or nail. You will need to use a toggle bolt to attach something in the same spot. If it is imperative that you have something in that exact spot, you can cut open the wall and insert a 2x4 or other piece of wood behind the drywall and then patch it up. That way you can put a longer screw into the wood for a solid support.