If the finish on your kitchen cabinets is in relatively good shape, but you want to restore the gloss, you can either polish it with wax or polishing compound, or you can give the cabinets a new coat of finish. Either way, wet sanding the finish first is a recommended procedure. It cuts down the sheen and levels the surface so that a new finish will adhere and polish will make a difference. You need wet/dry sandpaper for wet sanding, and the grit shouldn't be coarser than 220. You're not trying to remove finish -- just etch and condition it.
Wash the cabinets if they're greasy or dirty. If you need a strong grease-cutting detergent, mix 1/2 cup trisodium phosphate into 2 gallons warm water in a bucket. Use a sponge to wash the cabinets, then rinse them with clean water and dry them with a rag.
Remove the cabinet doors with a screwdriver, then remove the hinges and handles and store them in a safe place. Stack the doors against a wall and work on them one by one, laying each one flat on a pair of sawhorses.
Sand with 220- or 320-grit wet/dry paper, depending on the condition of the finish. Use the coarser paper if there are many surface defects, such as cracks or scratches. If the surface is relatively defect-free, use the finer paper to make a smoother surface.
Fill a bowl with water and rip a sheet of sandpaper in half. Fold the paper into thirds and dip it in the water. Start sanding in the middle of the surface on which you're working, moving the paper in circles and using moderate pressure. You don't have to worry about sanding with the grain, because you're sanding the finish, not the wood.
Sand the entire surface, moving the paper in whatever direction is most convenient. Keep the paper lubricated by dipping in the water. When you're finished sanding, wipe off the dark-colored residue with a rag and examine the surface. Go back and sand any areas you missed.
Wash the surface down with a damp rag and let it dry before you apply a finish.