There are three basic steps you'll want to take after you discover someone has hit your parked car:

Call the police
According to the Insurance Information Institute (III), an officer will document the incident and create an official accident report, which you will typically need to have when filing your claim with your insurance company. Be sure to ask for a copy of the accident report and the name and badge number of the officer assisting you.
Document the accident
The III recommends gathering as much information about the accident and damage as possible. This should include the location, general time of day and the weather conditions. If it is safe to do so, the National Association of Insurance Commissioners suggests taking pictures of damage to your vehicle, landmarks or signage that identify the location and any other damage at the scene, such as debris or tire marks.
Notify your insurer
Contact your insurer as soon as possible after the accident, as it will be easier to remember the related details, says the III. Your insurance provider will let you know what information is required to file the claim and what to expect during the claims process. Some insurance companies may also offer applications that allow you to file a claim and upload any photos of the damage through it. Talk to your insurance provider to see what's available where you live. And, keep in mind that if you were driving a company vehicle, you'll likely need to contact your business's insurance company.

If the driver who hit your parked car left a note, you should share the information they provided with both the police and your insurer. Typically, your insurance company will work with the other driver's insurer to settle the claim appropriately.