Depending on what you're facing, start out with some type of prep solvent, a grinder, a body hammer and general purpose dolly, plastic filler, some long sanding boards, progressively finer sandpaper (both wet and dry), some masking tape for when you prime your repairs. That's just if you have a few dings and need to block the truck down for paint. If you have more serious metal work to do, you'll need more stuff. A lot depends specifically on what you have to do.