Getting to Belize is easy. From the USA there are direct flights from Miami, Houston, Dallas, and Atlanta. From Central America there is one daily flight that arrives in Belize in the morning.
Once in Belize there are regular flights to San Pedro on Ambergris Caye. Please check with Maya Island Air and Tropic Air. Both local airlines can be booked online.
Also from Belize City the Caye Caulker Water Taxi offers regular runs to San Pedro.
From Corozal Town (border with Mexico) Maya Island Air and Tropic Air both have regular flights and the Thunderbolt Ferry Boat offers one trip a day.
Also, within Belize, charters both by air and sea are also available.