The quickest way to reach Ojców is by car. However you will need a good map and an even better navigator as it's easy to get lost if you don't know the area. 24 kilometres from Kraków, the journey takes about 30-45 minutes. There is lots of parking available in the national park, however most of it is paid parking, which basically assumes the role of a park entrance fee. Cheapskates can avoid the 10zł charge at Kazimierz Castle by parking in available spaces on the side of the road just outside the park (near Chapel on the Water for instance) and walking an extra five to ten minutes.
If you don't have a car, catch a bus from the private Unibus rank on ul. Ogrodowa just across from Galeria Krakowska. Tickets (5zł) can be purchased from the driver, and it drops you off at the parking area near Kazimierz Castle. For the most up to date bus schedules check the very helpful park website (in English also) - www.ojcow.pl - or at the bus stops at ul. Helclów and Ojców.