In the bottom menu bar, click on the button showing a burning house, the second icon from the left hand side, to get to the mission list. All available missions you can send units to are listed here. Right next to the name of the mission/incident sits a button labeled "Dispatch", click on it.
Now you are in the dispatch menu, here you can assign and dispatch vehicles to missions. If you are not sure which vehicle you need for a particular mission, scroll down and click the button labeled "Requirements for this mission". There you will find information concerning the mission.
Assign the vehicles needed to complete the mission and click on the green button labeled dispatch. Your units are now on their way to deal with the mission.