The Cancun airport is well-managed. After collecting your baggage and exiting the controlled area, you will see an organized area with many booths for activities and transfers. They are all in the same uniform. If you have pre-arranged transfers, look for the booth both inside and outside the main building. Most of them are legit companies, just beware of con-artists who usually are working alone. When in Cancun use the bus system. It is safe, very cheap, runs 24/7 and almost everywhere. It will not take you to the airport though. You will need to get a taxi or set up other arrangements with a private company.

Try to prebook your transportation before getting to Cancun since you will save some money with that, and always check that the person picking you up is actually from the company you booked your transportation. 

There are several well known companies that offer Airport-Hotel-Airport transfers. like: 

And another option of course, is paying the transportation from the airport to your resort destination is to have it included in your package stay.  As mentioned before, do NOT stop for anyone until your find your resort company representative which is always located OUTSIDE the airport terminal.  Not only will you find time-share hawkers, but you will find people willing to "help" you or may even represent themselves as a member of your resort company.  They will literally grab your luggage from you, walk you the 15 feet and hold it until you pay them.

When boarding your transportation, they may or may not wait for additional passengers.  If you are staying further from Cancun (perhaps in the Riveria Maya section or Playa del Carmen), realize that the driver will stop at every resort in Cancun first. If you don't want to delay the start of your vacation, you may offer to tip the driver extra if he leaves when you are ready.

This type of transportation gauntlet manoeuvering is only upon arrival - your departure from your hotel will be a breeze.

Also, plan ahead of time your  activities in Cancun, if you use any of the above companies websites you will save good money and time when purchasing your  Cancun excursions

For more information on travel requirements, contact the U.S. embassies of the countries you're visiting. A complete list of U.S. embassies worldwide can be found at the Department of State Web site.