Stay Calm
The first step is to stay calm. It will probably be the hardest thing to do but by staying calm you will be in a better position to help others and remember key details that may be important later to your insurance agent or lawyer. Staying calm will also help you to physically document details.
Emergencies First
Next, before you start any documentation of damages call 911. Then see if there is anyone injured that needs help immediately. Also, if you are in an area where it is dangerous to leave your vehicles you will need to try and move them if possible, otherwise leave everything as-is for the police officer. Once you are able to make sure there are no immediate emergencies to tend to, go to your glove box and grab the disposable camera, pen and pad of paper you keep in there for the specific purpose of documenting damages in a car accident or crash.
Pictures and Written Documentation
Go ahead and start taking pictures as soon as you can before anything gets moved by the police officer or the other driver. After you feel like you have taken plenty of pictures, quickly jot down your remembrance of the accident. If there are any witnesses, get their names and numbers along with an account of what they saw. You also need to get the names and numbers of everyone involved in the car accident and their account of what happened. Don't argue with them if you disagree... just try and stay neutral and tell them you just want to make sure everything is documented for insurance purposes.
