As to where to go it depends almost entirely on your interests. Do you want to fish, snorkel, swim, dive? Would you rather see Maya ruins, hike, bird, go caving, canoeing? Are you interested in cultural tourism, ecotourism, or what?
Belize is not that big a country, only about the size of Massachusetts, but like Mass. it has a variety of ecosystems, climates and topographies.
The weather also is somewhat variable depending on where you go -- inland or on the cayes or in the far south or whereever.
What are you interested in doing?
Belize First magazine http://www.belizefirst.com/