There are many factors to consider before deciding what to do but generally speaking there are only a few critical responses. First, make sure you protect yourself from other traffic or traffic hazards in the area that may have led to the crash. Do not obstruct traffic more than necessary. Next, if the scene is safe, obtain all the information you can and call 911. Be sure to provide the location of the crash and the number of vehicles and occupants involved. Then, if it is safe to do so, render any kind of aid that you can to the injured.
Finally, remain on the scene until help arrives. Remember that you can be a great asset to the investigating officers if you inform them of what you know about the crash.