How to fire dispensers in minecraft?

To dispense them you need a switch of some kind. It could be a tile, lever or button and it needs to be near it. Simply interact with it and it will dispense.

Give the dispenser a redstone signal. If you give it a button, pressure plate, or lever it will fire only once per activation. If you put down a continuous signal, IE a redstone torch, the dispenser will fire until empty.