From U.S. check American, United and now Southwest for competitive prices.  Since announcing flights to Belize, Southwest has made flying to Belize increasingly affordable.
Southwest flights do not post in travelocity, or other fare searches, so look at their site independently for price quotes. 

Belize City:
All international flights will arrive at Belize City International Airport. Upon arrival, there are two options of air transport to other parts of the country.
Tropic Air & Maya Airlines both operate from Belize City airport or Belize Municipal airport..  Both airlines have numerous flights to San Pedro/Ambergris Caye, so check their web sites for exact prices and flight times.  Each airline also offers travel options to various areas throughout Belize.

Flight Options to San Pedro:
 At this time, the average price from Belize International to the island is $160 US, for round-trip flights. The flight time to the island is app. 15 minutes and depending on other passengers, it may stop in Caye Caulker.  You can save by taking a taxi ($25) to the Municipal Airport and flights from municipal airport are currently $100 US for round-trip flights.  A group of four or more could save a substantial amount, however, if only two passengers, it may not be worth the extra time and taxi fare.  

Flight Options to Placencia:
Flights to Placencia are also offered by both airlines and both Belize International and Municipal airports.  Prices are ranging around $180-$200 roundtrip.  This flight is approximately 45 minutes.

Belize International Airport:
Belize international airport is a very small airport and very easy to navigate.  Once arriving, you will be directed to the immigration lines which are clearly marked. Chances are your flight will be the only international flight arriving at that time, so just follow the line inside. You will have completed the  immigration forms on the inbound flight.  Those are given to the agents, passport will be stamped and you will be sent to baggage claim (directly in front of you, & only one, so no way to get lost or confused).  After picking up baggage, turn around and you are in the customs line.  Yes, that easy! There are two customs lines.  If you are on vacation and have nothing to claim for duty charges, go in “nothing to claim” line.  They will check your passport, may or may not ask to inspect your luggage and tell you to proceed.  You are finished!
 As you exit, there will be lots of porters if you need assistance with baggage.  They charge $1 US per bag.  As you exit through the doors, if you are renting a car or need a taxi to water terminal, exit right to leave the airport, or on the left you will see the small check-in counters for Tropic Air and Maya Air. If you have reservations, they will quickly process your tickets and you are finished.  If no reservations, they typically have room on flights and are very helpful in getting you out on the next flight. After obtaining your tickets they will direct you through to the terminal to depart.
 So, yes, the customs, baggage, immigration, arrival, departure, Tropic Air, Maya Air, gift shops....all in one area, so you cannot get lost or confused.  If time allows, there is one restaurant, upstairs Tropic Air check-in, the food is good and reasonable and a great place to relax if you have time to waste.  If you need a drink,  inside the terminal, there are several small gift shops, a coffee shop and ONE bar.  “Jets Bar” The owner is a great guy you must meet if you have extra time.  He will definitely welcome you to the wonderful country of Belize, (and inform you he was named one of the top airport bars in the world.) Chances are, he will sign the article and give you a copy as he tells you the story!
After picking up luggage, you will see a few duty free shops.  If you want to purchase alcohol for your condo during the week.  This is THE place to purchase it.  You will save at least 50% over island prices!!   There is no problem carrying it on either Tropic Air or Maya flights.

Check these sites for additional info and reservations: