I am not a professional first responder by any means, I'll be interested to hear what they may have to say. On the two traffic accidents I witnessed and was first on scene, I suggest: Go slow. First on scene can be first clipped by a fast Audi second on scene. Be safe. Stop well away from the accident, well away from the traffic flow. Blinkers on, put on a hi-res vest, put on eye shield, gloves, and leather gloves as you get out of your car. Walk back up the road well off the actual road, taking care of any traffic. Report in (call 911) - stay on the line until they say to hang up, even if you could be assisting. Be prepared - that means training in basic first aid and keeping a FAK with you in your car. Stay within your limits. Tell victims you have basic first aid training, you've called 911, help is on the way, can you help them. If they say no, don't help. Don't attempt the roadside thoracotomy no matter how bad it looks. The best you can do is apply pressure to bleeding, hold wounds stable, and calm victims, until the pros arrive. It could be gross - a woman I supported her neck for 45 minutes until she was evaluated and collared had a fair amount of scalp pulled away. Expect to stay for the duration - the firefighters didn't want me to let go her neck until they were ready, and it took a long time. Then there are accident / witness reports, and then maybe handshakes and a thank you for your help. Another woman on a moped had a grisly compound fracture of her leg and was in shock (I didn't travel with so much as a blanket then, never again). Don't move anyone unless there's physical danger to them. Tell everyone to stay in their cars until help arrives. Look for fire and other threats, but don't be afraid of it. If you have a fire extinguisher, its okay to take it with you as you assess the scene.

Know your limitations. Most of us aren't used to accident scenes, I get a bit of the shakes re-living these ones now. You won't remember to do everything, you have to be careful - you have to err on the side of caution, stay within your training, and do no harm. Help will be there as quick as they can.