1) call 911
Unless you arrived in an ambulance with a trained partner and are yourself trained then call 911 or equivalent first. The reason for this is that once you become involved in providing aid you will lose your time perspective and precious minutes will pass delaying the arrival of needed help. If it turns out that the EMS / Fire personel are not needed then they can be called and canceled later.

2) Make the scene as safe as possible.
If you arrive on a traffic incident you may put your car between the accident and traffic, put your flashers on and your emergency brakes. Then, if you have flares put them behind your car at 10', 20', 50'. If there is a sharp bend in the road then run back and place one on the other side of the bend so folks don't come whizzing around the bend only to discover that they cannot stop in time to avoid becoming part of the problem.

3) Don't become a casualty!
If there is a fire stand back and get some marshmallows - seriously! Go looking for anyone who may have been ejected from the vehicle and assist them. Don't become involved in extinguishing a fire. That is orders of magnitude beyond the training and gear you are likely to be carrying in your vehicle unless you happen to be a firefighter with one of the response vehicles. Unless you have turn-out gear and a mask don't approach a vehicle fire.
If you have latex gloves put them on. Just because they had a vehiclular accident doesn't mean that they don't have HIV, tuburculosis, or any other communicable desease and their blood will be all over the place just trying to infect you.

4) Don't harm the victims!
If they can stand up and walk about and are already doing it get them to sit down or better lay down away from traffic and accident. They may have spinal injuries that are not obvious to you or themselves. With a neck injury (very common in vehicular accidents) they may feel fine until they turn their head the wrong way then they will stop talking, breathing and standing as they sever their spinal cord in the neck and fall to the ground a quadruplegic unable to breath (think Chris Reeves) If they are in the vehicle and it isn't burning then keep them there until the ambulance arrives with the proper spinal imobilization devices - collar, board, etc...

5) If there is severe bleeding that you can reach apply direct pressure and bandage.

Without EMT or better training stop here and wait for ambulance.

As a trained individual there are other items to attempt and examine.

triage - apply MCI triage if there are more patients than there are trained aid providers. If you are alone then this means you will only be treating one patient and it had better be the one that has a chance. Those who can walk get no attention and those who aren't going to make it get no attention. Life is tuff.

ABC
Check all victims for breathing and if absent check their airway while maintaining spinal immobilization. (Many EMT's, myself included, carry cervical collars in their private vehicles) the airway needs to be checked and cleared / stabilized prior to applying a collar.

Check rate and quality of breathing and assist if necessary.

Check rate and quality of circulation - this involves checking for major bleeding, checking rate and strength of pulse and blood pressure.

When the Ambulance arrives have the MCI triage results ready and the ABC assessment of the individual you are working.