You can report unlicensed vehicles online through the DVLA website, see website in related information.
You can also contact Police Scotland. In some areas you will have to contact the local authority direct as they have a responsibility to remove such vehicles. In other areas you need to contact the police first who in turn will contact the local authority. Your local police station will be able to advise you.
In all cases you will need the make and model of the car, the registration number and the location.