How can i find car owner using car reg please?

Police or Vehicle Licensing authority DMV (USA) or DVLA (UK) or insurance company (if known)

All will want to know your reason for wanting the information and may refuse to give it if they do not think you have a valid reason for knowing.

Owners of cars by there registrations is not public knowledge to prevent crimes it would be a criminal offence to obtain information as to the owner of a vehicle or their home address without their consent unless for insurance or legal reasons other than through the above authorities.