There are many factors to consider before deciding what to do. Generally, there are 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 as much information as possible 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 to the injured.
Finally, remain on the scene until help arrives. Remember that you can be a great asset to an investigating officer to inform the officer of any information you have about the crash.