To add trucks for your company, you will go to "Settings" → "Trucks" → Select the button "Add a New Truck".
When adding a truck, the only required field is the Name of the truck (see below). This can be a truck number or name, basically whatever you use to refer to that truck.
The rest of the fields are optional, but we do recommend filing them out so that you have the information on hand if you ever need to reference it! And as always, don't forget to "Save Changes" when you are done.