O.K. we will assume it is a standard 2 or 4 place aircraft with two people that you can see. If possible approch the area from the down wind side, When you leave your car take your fire extinguisher and first aid kit. more than likley it will have low lead gas on board, smoke is bad, if it is coming from the eng area send a squrt from your extinguisher in the holes just behind the prop. I am going to assume(again) that you have a dry chem ext. If there is fuel on the ground dust it with the ext. Now check on the passengers be very carefull of the head area (any thing in the back seat that was not tied down could of hit then in the back of the head)
hopefully because of your actions there is no fire. insure they are breathing make sure there head is not on there chest and cuting off air flow. leave them in the aircraft and use your first aid kit to stop or reduce bleeding. During all this time you have to make sure some fool dosen't come running up and try to Pull the people out of the wreak. When you are alone you have to be a firefighter, medic and cop. buy this time I hope a LEO and the fire department is there to help you. When they get there back off and let them do there jobs, but stay around they will have a lot of questions for you.