The historic city center of Regensburg is separated from the northern banks by the Danube. "Oberer Wöhrd" and "Unterer Wöhrd" are inslands in the middle of the river that belong to Regensburg as well.
The central square of the historic city is the "Alter Kornmarkt". The Maximiliansstr. that starts at this square leads southwards to the main station.
Cars and Motorbikes
A large part of the historic city center as well as the Stone Bridge cannot be accessed by private cars or motorbikes. In addition, carparks are quite expensive and rare, that's why it is recommended to park somewhere outside the center and walk there or use the bus.
As the historic city center where many of the sights are is quite compact and small, it may be the best option to walk.
Between the main station at Albertstr. and the central square "Alter Kornmarkt", there goes the "Altstadtbus" quite regularly. There is a variety of tickets you can choose from, i.e. single tickets, stripe tickets and all-day tickets.
Of course, there are also bike rentals where it is possible to hire a bike. Some of the places even help you planning a bike tour along the Danube for example.