If you are staying in Cancun, or Playa del Carmen, you can visit Tulum as a one-day excursion. Many agencies in these resort towns offer tours to Tulum. You can also get here by bus or mini van (colectivo), or by renting a car.
If you want to stay in one of the nice beach hotels in Tulum, you can fly to Cancun's International Airport (CAN), and rent a car to get here. Alternatively you can catch a bus.