When you find that your car has been stolen, the first people to call are normally the police, or other law enforcement agency for your area, and your auto insurance company. In most states the police should enter your vehicle information into national and state auto theft computer records.
According to sites such as the New York State DMV, by law enforcement entering in your vehicle's information into the theft computer records, the theft should be noted on your vehicle title record and thus help prevent someone from selling the vehicle or applying for a title.
To make certain that the police has entered the data into the system; it would be a smart move to contact the Department of Motor Vehicles as well and inform them of the status of your vehicle.
If your vehicle is recovered, make sure the police cancel the stolen vehicle report/alarm so the recovery will be listed on the title record. You will not want to use the vehicle or apply for new plates/registration (if needed) until you are sure the alarm has been cancelled. Again you might want to call the DMV to double check their records.
Beyond informing the police and DMV, make certain to let your auto insurance company know about your car being stolen. If you have comprehensive coverage on the vehicle you can make a claim with your insurer and they can begin working on the claim.
10 Ways to Prevent Auto Theft is an article in our Learning Center that might also be of help.