Open your owner's manual to the front page or to the pages with instructions for your radio. If the code is located in your manual, it usually appears on a small sticker on the inner cover or in the radio information section.
Check your glove compartment for a radio code card or sticker. Most of the time, the code is in the owner's manual, but sometimes the code is in the glove compartment.
Call the radio manufacturer to obtain the radio code. This requires you to have the radio's serial number so the manufacturer can give you the correct code.
Call the dealership you purchased the car from. It may have the code if your car is new.