You do not need to exchange money in Belize, US Dollars are accepted everywhere. The exchange rate is a static 2 BZ$ = 1 US$.  Small bills ($20 and below) are most commonly accepted and bills should be in good condition (no rips or tears).

There are ATMs in the village where you can get additional cash.  Check with your bank to confirm any additional international fees or limitations.

Mastercard and Visa are accepted at most restaurants, stores, and tour companies though some may have a minimum spend required.  American Express is accepted at limited locations, and Discover is not accepted in Belize at all.