IF it's just to get rid of old polish and oil to freshen up a sound surface, a good wash down with sugar soap, dry, work over it thoroughly with a rag soaked in turps or white spirit. Turn the rag and rinse it out a lot, don't skimp with the turps-- the object is to remove dirt and grease, not smear it around.

Make sure you get into all the corners and cracks-- if you don't Murphy's Law states that there will be some horrible black stuff in one that your brush will pick up and spread all over your nice new varnish.

Once that's done, rub down with sandpaper on a block, wipe down with a tack rag (A sticky cloth available from paint merchants) to pick up and remove all the dust, then apply varnish per makers instructions. Note "rub down" not "sand off." The object is to get rid of any old brush-marks or light surface damage and provide a key or "tooth" for the new varnish.

If you're talking about actually removing the old varnish, the only route is nitromors or some other chemical stripper. You dare not use heat or heavy sanding, especially in an older boat where you don't know if someone else has perhaps already done this, leaving a paper-thin veneer.

If there are places where the teak veneer is damaged, or you do sand through, don't panic. Small sheets of teak veneer are available from any decent craft shop or online, and larger pieces from high-end timber merchants, the type that service the furniture trade. It's not terribly expensive. Cut round the damaged area with a a really sharp blade at 90deg to the surface then carefully remove the old veneer with a chisel sharpened as well as you can get it, cut a new piece to fit and glue it in (remember to match the grain direction.) Pros cut nice wee oval patches but that's just showing off, a diamond shape looks fine. Sand flush and finish as above.