If your vacation package includes transportation.
You'll receive instructions on catching a taxi from your travel representative (usually they have a desk at the hotel). In most cases you'll need to check in with them at lease 24 hours before departure to reconfirm your flight, and make arrangements for a taxi to pick you up at the hotel. Ask which terminal your flight departs from.

If you need to get your own transportation.
The conciere in the hotel lobby will get a taxi for you. No need to plan too far in advance, there are usually taxis waiting in the hotel driveway. Many airlines ask that you reconfirm your flight with your travel representative, at least 24 hours before departure. When you do this, ask which terminal you depart from. If you are heading to the airport and don't know which terminal, the driver will help you. It's typically the same terminal you arrived at

Checking-in.
Once you arrive at the airport, someone with a cart will come to help you with your bags. Let him! He already knows which flights are departing, and where the counter is. Once you see the massive crowds of people, and get into the confusion of a Cancun airport departure, you'll understand why his tip is a good investment.
When you are in line, get your documents ready. You'll need your ticket, tourist card, birth certificate, and drivers license.
Security is very thorough in Cancun. In each line, just before reaching the check-in counter, agents will inspect EVERY bag. This includes the bags you are checking, and your carry-on.
After the inspection, it's check-in as usual.

Waiting for your flight.
MAIN TERMINAL: Here they have a wide selection of shops, and places to eat. They include: Burger King, Domino's Pizza, TGI Friday's, Baskin Robbins, Corona Beach Bar, Grab & Go, Guacamole Grill, Hasta la Vista, and Khalua Cafe. The prices aren't cheap, A Whopper combo is about $6.50US. Nachos and a beer at TGI Friday's is about $7.50 US. I'd suggest eating here. Once you've moved up to the gates, the selection isn't as good, and the prices seem even more outrageous.
The walk to the area where most international flights depart from (known as Gate A) is the original terminal in Cancun. All the additions made to the airport over the years, and made the route to the original terminal rather challenging. There are twists, turns, an up escalator, then down, and yet another up. It's well marked, so you'll find it, but allow plenty of time to make the walk and clear security. Security is your standard walk-through metal detector, with x-ray scanners for your baggage. You'll have to show your boarding pass here.
If you want to make a last minute liquor purchase, we've found that the duty free shop right after security has pretty good prices. Look around at all the shops upstairs at in the Gate A area, you can always walk back down the steps to this duty-free store.
You'll have one last inspection before boarding the plane. They'll check everyone's carry-on, and make a few people remove their shoes, etc. Then it's on the plane for the trip home.

CHARTER TERMINAL: Dining selections aren't nearly as good as at the main terminal, and of course, prices are in the category of "Airport Expensive". There are a few shops upstairs near the gates, but it won't take you too long to see everything. After checking-in, you'll take an escalator up to the second floor and go through security. Then go to the waiting area for your gate.
You'll have one last inspection before boarding the plane. They'll check everyone's carry-on, and make a few people remove their shoes, etc. Then it's on the plane for the trip home.