Your first priority after an accident is to ensure everyone’s safety. If possible, to do so, pull over to a safe spot on the side of the road (as near to the scene as possible) and check to see if anyone is hurt. Call emergency services if necessary, and also call the police to file an accident report. Do not leave the accident until the police have arrived and finished their report.
If you are in a condition to do so, you will also want to gather the following information:
The names and relevant personal information of those involved (license plates, contact info)
Insurance information + policy numbers
Photographs of the scene
The name and contact information of witnesses
While it is not absolutely necessary to gather this information yourself, as a professional investigator will also likely try to gather this information, it can nonetheless be useful in making sure that evidence to support your claim isn’t lost in the bustle following an accident.
When you are allowed to leave, you will want to contact your doctor in order to be checked and treated for injuries, including those you may not be aware of. You will also want to contact an injury attorney, to help you form your case and to assist you in dealing with insurance, and your insurance to alert them that you have been in an accident.