If you don't know, you may want to take a look at this. This link is to the ACC Accident Reporting Kit. Included is a 3 step checklist that you should perform if you are ever in an accident, a supplies list to help you document the accident, and some easy to use diagrams and forms for illustrating the accident.
Remember, this is information that you should have before you get in an accident. So print it out, get the supplies, keep them in the glove compartment in your car, and drive safely.