the main thing you can do in a crash situation is to make sure they are breathing first and for most. if not you have to do somthing as death with shortly follow, clear airway and perform cpr. (2 compressions followed by 30 breaths and dont stop once started until thay start to breath or you physically cant go on cause it is hard work)
if they are breathing and not in imminent danger leave them in the car/position and reasure/talk to them until the fire/paramedic/ambulance arrives you could try to get basic info from them to pass on. name/age/speed/medication/how it happened etc. most crashes will result in trauma and internal injurys and as a result of moving without proper spine immobilization or care of internal injurys will cause more damage.