When you are in a car crash there are five things you should keep in mind.

1. First and foremost make sure you are safe.  If you can move get off the roadway, never stand between parked cars, check for downed power lines--generally make sure you are out of harm's way.
2. After you look after yourself and if you can, help others.  First aid is most important to save lives.  If you are trained in CPR do it!  Anyone can wad up a rag and press firmly of bleeding limbs.  If the injured person is trying to get up or get out of the car caution them to remain and stay still--this is to prevent any further damage to the person's spine.
3. Call 911--or make sure someone else has called 911 to initiate the emergency response system.  Make sure help is on the way.
4. One things settle down and everyone is safe and 911 has been called try to gather the names and contact information for witnesses.  Oftentimes people do not want to remain at the scene of a crash, they do not really want to become involved unfortunately.  You need to act fast before people leave the scene of the crash.
5. Finally, get out your cell phone and take pictures, a panoramic picture is always helpful.  Take a video of what the scene looks like, if permitted, keep the video rolling as treatment is rendered or  police show up.

Bottom Line: Keep your wits about you when in a car crash and keep safety first and foremost in your mind.  Once you and others are safe call 911 and start taking pictures and videos of the scene and as the scene unfolds while treatment is rendered.