There are plenty of buses that head to Valladolid, Playa del Carmen, Cancun, Coba, and many other places in the Yucatan peninsula. There are two bus stations: one for immediate cities/towns around the Mayan Riviera coast, and the other for inland destinations such as Valladolid.
To reach Tulum it's easy. You can take a bus or a taxi from Cancun or Playa del Carmen. Otherwise you can easily find a lift waiting on the main road. There are many buses that run through the Riviera Maya from Cancun to Tulum. Surely they'll stop when see you make a wave. This can be cheap and fun!
To reach Tulum, you will have to take the ferry from Cozumel or a bus from Cancun, to Playa del Carmen. The bus terminal is easy to find within a couple short blocks from the municipal pier. Check the schedule, if you can, ahead of time, because there are only a few buses each day. Fare is cheap. Bus is definitely the preferred way to go. If you should find yourself short on time and opt to rent a car, stick with the major rental companies and if possible drive the type of car you're familiar with. Car rentals are relatively expensive, and may not prove to be reliable.