If you want to visit the iconic dome of St. Peter's Basilica during your visit, you'll need to select the correct tickets to do so; standard St. Peter's Basilica tickets do not include access to the dome.
You can visit the dome by selecting one of the excellent 'Dome Climb' tour options, which include a fully guided tour of the basilica as well as the record-breaking dome. You can even choose to visit the sacred crypts below the church, where saints (including St. Peter himself) have been interred over the centuries.