Check to see if you or any other occupants of your vehicle are in need of medical attention. If so, call 911 immediately. Turn on your emergency flashers. Make sure that it is safe to exit your vehicle. If you can safely leave your vehicle, check to see if the occupants of the other vehicle(s) are in need of medical attention. If so, call 911. Contact the proper law enforcement agency and report the crash.
While you are waiting on law enforcement, preserve as much evidence as possible. Use a camera or smartphone to take photographs of the vehicles before they are moved! Also, photograph damage to vehicles and any other property damage caused by the collision. Make sure that you photograph the license plates of all vehicles involved, write down the names and contact information of any witnesses, including the driver of the other vehicle(s).