If you're driving to Roskilde, you should be set. Parking is readily available and driving is the option that will give you the most flexibility if you plan on visiting the outskirts of the city and seeing the National Park Skjoldungernes Land.
However, do not worry if you don't have a car available. Roskilde is a pocket-sized city and the Roskilde Cathedral and Viking Ship Museum can be reached by foot within 10 and 20 minutes accordingly. For train-planning, use the Journey Planner, which is easy and convenient.