Cancun is about 100-120 min driving (120 km) north of Tulum. As you come out of customs, you will have about a million people and stands asking if you need a taxi, rent a car or private transfer. 

ADO has large buses that take you from the airport to Tulum. They will charge you around $14USD to do the two hours drive directly to Tulum. The ADO bus company has air con, comfortable reclining seats and toilets. 

The other options are either to get a private transfer from the airport or to get a taxi. Just ask us and we can organize it for you.