From the Belize airport you need to take a taxi to the ferry station. The taxi costs roughly $25 USD or $50 BZD. *keep in mind that you can use US dollars in Belize.
Once you get there you need to buy a ticket and, if you have larger luggage, check it in before you board the boat. You’ll also need to check yourself in once you’ve bought the ticket (this is done at the window directly next to the window you buy your ticket out so it isn’t too difficult). The ferry leaves almost every hour so you shouldn’t have trouble getting on one once you get there.
If you’re traveling on a REALLY tight budget you should take he Ocean Ferry rather than the Express Ferry since it’s a couple dollars cheaper. Just be sure to tell your taxi driver to take you to the Ocean Ferry dock. *almost everyone speaks English in Belize so you shouldn’t have any problems with communication.
When you arrive on Caye Caulker there’ll be taxi golf carts lined up ready to take you to your hotel/hostel. However, keep in mind that the island is tiny and that most likely your hotel/hostel is less than a ten minute walk from the pier. As long as you have a general idea of where it’s located you should be fine walking.