Leave a note saying "Sorry for hitting your car, here's 20 dollars to make you feel better", Then, put 20 dollars on the back of the note. Also, if you do not want to be charged with a hit-and-run, leave your contact information laced with lots of smiley faces.
Run
iou
Write down your name, insurance information and number and leave it underneath the windshield wipers.
leave your information and take lots of pictures. If there is damage, they will contact you. Call the police
Try to find the owner. Leave a note with your contact info on windsheild worst case...call a claims associate.
Call the police, they can contact they owner for you and help you get everything ironned out.
Leave a note on the windshield with your contact info
Give the owner your name, address, and tag number in person or in a note attached to the object that was hit. Report the crash immediately to the proper law enforcement agency.
Leave a note with your name and number if you can't find the owner.
From your question I am perceiving that you parked straight and someone came and parked beside you but the rear of their car is blocking the rear of your car. You would be at fault because you should have realized that you wouldn't have been able to back out. In that case you should have contacted the local police (don't call 911) to have them do something with the car if you are not able to find the owner of the vehicle.
leave note on vehicle
You should leave a note with your name, number, license number and insurance policy number and information. You may have been caught on camera so you shouldn't assume you will get away with it.