Once you are in Canterbury, just walk. Anything you want to get to will be within walking distance, cars are just a nuisance, and if you walk, you will be able to take in the amazing architecture and capture the flavor of the city.
Although you can take trains to Canterbury, the buses take you to close to the centre of the city. The trains are a bit more of a walk. If you're coming from anyplace in the Kent area, even London, you'd be better off taking the bus. Any longer distance, go with the train.
You can take the train to Canterbury -- BUT -- there are two train stations on different lines -- AND neither is within easy walking distance of the center.
The GOOD news is that, once you are in the center of town, what you want to see is pretty close at hand.
Getting around by car in the center isn't advised -- as new cars and old gates don't blend well!
Canterbury is easily accessible from London either by train or National Express coach from Victoria. If you go by coach you can stop of at Bluewater (Europe's largest shopping centre) and pick up some bargains. Alternatively some services go right round the coast Ramsgate, Margate, Herne Bay.
The canterbury park and ride busses leave from the sucure carparks every 15 mins or so, they use single and double decker busses