PADI has an Open Water Diver manual that is available from any PADI dive center worldwide. Look at www.padi.com
if you don't know where there is a center. If you arrive having done no preparation expect the course to take 3-4 days. What some people do who don't have the time for a normal referral is to read the manual and do all the exercises in it. You should be able to find a dive center in Belize that will mark the "knowledge reviews" if you fax/ email them, in anticipation of you then doing the course with them.