I would NOT use a belt sander, they are far far too aggressive for veneer, even with the finest belt. They are not a tool for a beginner, IMO. You would get better results from a Random-Orbit sander and 240 or 320 grit. Take your time, watch carefully for the veneer being worn through, especially at the corners.
I would then use an acrylic varnish (water based) or if it's a high wear site (bedside tables are terribly open to cups of hot tea) a hard-glaze polyurethane. If it's in a wet area, or drinks get put on it (i.e. a bar top), you might find a formaldehyde resin is good, see Andy Dingley's post on varnishes in another thread.
There is also the problem of matching the newly sanded top to the uv- darkened carcase. You may need to use a stain before varnishing, and experiment on a bit of wood that isn't visible to get the right density/ colour.