Be sure it's in good shape and functioning well. And check to be sure you have all the dust bags, special wrenches, and attachments. The machine is powerful and, if not used properly, can quickly gouge your floor beyond repair. You may want to practice on a piece of plywood, or with fine sandpaper, until you get the hang of it. Always use a dust mask.
Ear protection is recommended as well.
Coarse grits will be needed for the rough sanding and for removing the old finish. Finer grits will be needed toward the end of the sanding process to provide a smooth finish. If you need to remove paint or to sand cupped boards, start with a 20-grit paper. To remove shellac or varnish, a 36-grit will do. The actual number of sanding passes, from 2 to 4, will depend on the condition of your floor and the build-up of old finish and wax.