You need to have an existing reservation to ride the bus. If you haven't made one upon arrival at your resort, you can do it while there. Just be sure to do it at least a day before departure.
Then, you will have a piece of paper, giving you instructions as to what time to arrive at the bus stop. Bring that with you. The driver will collect if from you as you board the bus. That paper will be left either in your room, or under your door the day before you depart.