Locating the owner of a license plate for free is not a simple process. Many websites allow you to retrieve the information, although they will charge for the service. The only way to access the information for free is using the Department of Motor Vehicles in your state, although it will only provide you with the name of the person and not his address. You will need to do some sleuthing to locate an address.
Look for clues in the license plate itself. If it is a personalized plate, can you draw any clues from this? The license plate can refer to an organization or a business that you can ask for information. Provided you have a legitimate reason for doing so, such as wanting to buy the car, a company may give you the information.