Vehicles are added automatically. When you add a sensor to a new vehicle, the system will add a vehicle entry upon completion of your first drive.
If your car reports is VIN (Vehicle Identification Number) over the OBD port, then Dash will likely be able to determine its make, model, year, and important details like engine size. In that case, the you'll see an entry with a name including the make and model, like "Toyota Camry added at..." In this case, most of the details will be pre-filled, and you can generally leave the vehicle details as-is, but it's a good idea to review them. You can also customize the vehicle name and picture.
If your vehicle doesn't report its VIN over the OBD port, or if you are logging a trip without a device, then Dash will create an stub entry with a name that doesn't include the vehicle's make and model. In this case, you should edit the entry so it has the correct make, model, year, engine size, and fuel tank size - these are the basic fields that affect Dash calculations.
At the bottom of the vehicle detail page, you'll also find the option to delete or merge vehicles.