The currency system in Belize is fabulously easy for American visitors.  The Belize dollar is locked at $2 Belize = $1 USA.  So it's very easy to see how much something is costing in USA when you go shopping.  Most accommodations and tours are listed in USA$ prices. In shops, assume that all prices are in BZ$. Almost all restaurants price in BZ$ but some of the fancier, foreign owned resorts may have USA$ prices on their menus.  Foreign exchange at the banks has a lower rate for selling and a higher rate for buying.

Nearly every place (both inland and on Ambergris Caye) readily accepts USA$ currency.  However, don't bring large bills (anything above a $20) because it could be hard to use $50 bills unless you spend $45 of it. Make sure that the USA$ currency you bring is in good shape as if it has tears or marks on it, you may find that it's refused.

A lot of hotels, resorts, restaurants and businesses, particularly in the main cities and towns accept credit cards now.  However, in the villages it is handy to have cash on hand for small purchases.

The easiest way to obtain Belizean currency is at automatic teller machines.  These will be found at banks in Belize City and all of the District Towns.  Additionally, the occasional gas station on major highways, or supermarkets also will have an ATM.  A North American bank or credit union debit/ATM card with a four-digit PIN will work at all of the Belizean bank ATMs.   The currency dispensed will be in Belize dollars, of course. 

 Be sure to exchange your excess Belize dollars  prior to leaving the country, as it's almost impossible to change outside of the country -- even in adjacent border areas of Guatemala and Mexico.  There is a bank at the international airport's departure lobby (before you pass through passport control), and money changers work at the land borders.