How do I refinish an old nightstand to a high gloss?

Was it stained or painted previously? Usually you can just paint it the type paint you want if so.

Wood?

You'll have to start by sanding off the existing finish. Get the wood back to a raw state and then you can go about applying your new finish.

It will depend on the existing finish currently which you may have to sand or strip off. If it's a laminate, you'll need a bonding primer to paint over it. If it was previously painted, you can paint over it.

I converted a 1970s octagon nightstand into a dog bed. It had a lot of nooks and crannies, so I didn't want to have to sand it. I went to ACE hardware and bought a spray can of primer. I bought outdoor paint meant for shutters and trim. I wiped the nightstand down with warm, soapy water and let it air dry overnight. The second day I put a sturdy cardboard box down, covered it with a tarp, and set the nightstand on top so that the bottom of it would not be touching the floor. I sprayed it with 2 coats of primer, letting it dry an hour between coats. The third day I brushed on 2 coats of the shutter paint, and it is beautiful. The shutter paint is glossy enough for my taste, but you could probably add a coat of poly if you still wanted more gloss. Good luck!

Just remember that the glossier the finish, the more it will show any underlying imperfections and any future wear and tear.

High gloss water base polyurethane oil base will yellow over time. I did a wood nightstand back in 70's and the high gloss is still shiny to this day here's some tips https://www.woodmagazine.com/woodworking-tips/techniques/finishing/polyurethane Here's some videos for more help https://www.youtube.com/watch?v=96KywJlZ9g4 MORE https://www.youtube.com/watch?v=PpUBzF0LSnM ANOTHER