how can I restore my dark wood table top?

go to www.usu.edu [Utah state university ]
click on search usu enter Sanding click on top one sanding for pdf on how to resand wood very helpful although long winded. I'm saving this pdf.
There is quite abit of info on sanding just by searching the internet
Or you could follow what I did below.

Yep did mine some time back.
Mask, Lint Free cloth, White spirit, coarse medium and fine sanding sheets for your electric sander. Varmish brush, Polyurethane varnish of course, Sanding block.

Do the following after each sanding
Vaccum off and wipe over top with lint free cloth and white spirit after each sanding.
All sanding operations should be done With the grain.
ACTION:

1. Sand off all varnish with coarse sanding sheet.
follow with medium sanding sheet.
lastly fine sanding sheet.
Don't forget to vac off and wipe down after sanding.
Apply thinned first coat of varnish allow to dry for couple hours then lightly rub over with fine sanding paper preferably on a sanding block.
clean off as above allow white spirit to dry.
Apply further coats in the same manner< lightly sanding each coat] Once you feel happy with finish coat obviously don't sand it and job done as new.

Bob
September 2010