First thing you do is make sure everyone is okay.  Next, call the police.  They will send someone out if there are injuries.  Keep in mind, a police officer will not be dispatched if everyone is okay.  If the police are dispatched to the accident they will take statements and call for medical assistance if needed.

If it is determined that an officer is not needed, you will need to move your cars to the side of the road.  You will need to exchange information with the other party.  Try to stay calm and focused.  Exchange names, addresses, phone numbers along with insurance policy numbers and claims numbers.  If there is an officer taking information be factual.  If there is no officer look for witnesses and get their name, phone number and statements.

Contact your insurance claims department and report the accident as soon as possible.  If you are unsure of the phone number or the procedure contact your agent.