Your best bet is to try to contact the owner. Start by trying to find the VIN. The vehicle identification number is usually registered with your local DMV, and they should be able to use the VIN to contact the owner.
Each state’s laws are different, but local authorities almost always have to be involved with this process. In some cases, they’ll come out and tow the vehicle. If no one claims it, the car will go to auction, and then you can bid on it.