You can report unlicensed vehicles online through the DVLA website, see website in related information.
You can also contact your local police (via 101) although the procedure will vary from force to force. In some areas you will have to contact the local authority direct as they have a responsibility to remove such vehicles. In other force areas you need to contact the local police force 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.