A complete answer would be a book, not a quick comment.
A lot depends on what the finish is, French Polish, Oiled, Polyurethane Varnish, Cellulose, Acrylic etc. etc.
The only thing I would recommend you try before you get professional help would be a gentle rub over with a car type rubbing compound- T Cut or similar.
If it needs more than that, it's beyond amateur repair
Cheers
Dave