The CDC recommends a number of vaccinations before visiting Belize, in addition to a course of anti-malarial medication. Some people on my trip took their chances without either of the above and they were fine. I preferred the extra comfort and would recommend seeing a travel doctor a few months before leaving.
English is the main languages spoken in Belize. You'll also find some Spanish and number of local Mayan dialects.
The US dollar is accepted everywhere, so there really isn't any need to convert your money. The conversion rate is 2 Belize dollars to 1 US. Credit cards are widely accepted. ATMs are not plentiful.
Expect to pay about $40 US as a departure tax when leaving Belize.
Punta Gorda roads are paved, but you'll quickly find dirt roads as soon as you leave town. In the morning, this isn't a big deal. After a long hot day, bouncing around in a car or van for the last 20 minutes back to your hotel can be a bit grueling. (I'd also discourage renting a car for this reason. If you do rent, upgrade to a truck or something with more off-road capability.)
The climate in Belize is hot and humid. Bring extra shirts or polypro tshirts that can be washed in your room and will dry quickly. Make sure you have plenty of water when you leave for the day.
If you are staying outside of Belize City, pack any necessities that you may require. Even in Punta Gorda you may not find simple items you'd expect at home.
Bring a water camera! I did not pack one of these and would have loved to have one at the caves.
I'd recommend packing some Nu-Skin in case you get any scrapes or cuts. On the cave tour, I skinned my knees which would normally be a minor inconvenience. With the humidity, it just didn't heal well. Luckily, someone had packed some and was willing to share.