Check to see if anyone needs medical attention. If so, call 911 immediately.
Turn on your emergency flashers, and if you wish to exit your vehicle make sure it is safe to do so.
Call the proper law enforcement agency and report the crash. While waiting on law enforcement, make sure to document as much evidence as possible:
Use a camera or smart phone to take photographs of the vehicles before they are moved.
Photograph damage to vehicles and any other property damaged caused by collision.
Photograph the license plates of all vehicles involved.
Write down the names and contact information of any witnesses.
Provide any information requested by law enforcement, but do not speak to any other persons (including the insurance company) about the crash until after you speak with an experienced personal injury attorney.
Seek medical attention as soon as possible after the crash. Do so even if you believe you are alright. Many injuries do not materialize until days after the accident.