You'll probably see St. Peters Dom from a distance before you actually find this incredible building. Rest a while inside if it's open or just admire the facade from outside.
You'll be spoiled for sightseeing choice when spending time in Worms. St. Peter's Dom is a great place to begin here, but there are numerous other fine attractions to experience. Nibelungen Museum, Tiergarten Worms, and Jewish Cemetery are close to the principal church and and will cap off your day nicely.