Get a bus to Flores where you can change to one of the express buses to Belize City.
Fuente del Norte, Linea Dorada, and San Juan Travel Services (Guatemalan companies) provide Express bus service between Flores and Belize City. In Belize City, buses arrive at the Marine Terminal on North Front Street near the Swing Bridge, and are not scheduled to stop anywhere else in Belize. Buses operate on Sundays and public holidays. Not all buses are air-conditioned. The agent in Belize City for these bus lines is Mundo Maya Travel located at the Brown Sugar Market Place on North Front Street, Belize City. Phone: (501) 223 1200 / 663 5957.
Flores to Belize City (235 km / 146 miles / about 4 to 5 hours)
Fuente del Norte departs daily at 5:00 am; US$25.00
San Juan Bus departs daily at 5:00 am; US$ 25.00
Linea Dorada departs daily at 7:00 am*; US$ 25.00
* Departure of the Linea Dorada bus is sometimes delayed because it waits for a connecting bus from Guatemala City.
I've been told the Fuente del Norte bus is the best bus. The LD is a big 48-seat(?) bus, but not always airconditioned. The San Juan bus is a smaller 20-seat(?) bus, and from what I've seen lately, looks like they've got a new one.
I haven't updated the departure times for a while, so you might want to confirm them.http://belizebus.wordpress.com/