It depends on what the finish is. If polyurethane, you'll probably need to use a heat gun and plastic scraper, taking care not to burn the wood or gouge it. Poly isn't very susceptible to paint stripper, so that's not a good option. Most modern finishes are some variation of poly.
If it's a lacquer finish, paint stripper will work very well. I have a plastic tub that I lay the body in, paint it with a thick coat of stripper, snap the lid on and walk away overnight. Then use a plastic tool to scrape off the finish.
In either case, light sanding will probably be required, but I don't recommend sanding to remove finish, as you're very likely to change the contours of the body or damage it by excessive wood removal. Sanding is also very labor intensive.
Whatever you do, take your time, be patient and careful. You can screw up a body pretty easily.
And the most critical step is doing meticulous sealing and sanding before you paint! Any imperfections in the body or the sealing process will show up in the finished paint job.
Also - read this thread: Threads describing How To Refinish a Bass