You can take a bus from Cancún to Playa del Carmen, which takes about an hour and a half. From Playa del Carmen, you can take a ferry to Cozumel, which takes about 45 minutes. The cost of the ferry is approximately $15 one way.
There are also direct flights from Cancún to Cozumel, which take about 20 minutes. The cost of a flight varies depending on the time of year and the airline.