Are you hurt? Is anyone in your car hurt? Does it appear (without getting out of your car) that anyone else has been hurt? Before you do anything else, determine whether anyone requires emergency assistance. When in doubt, call 911 and ask for an ambulance.
Move to Safety
If the accident was a minor one and there appears to be no serious injury, move your car to the side of the road and away from traffic. If you're injured or your car cannot be moved, you and your passengers should stay in your car with your seatbelts on. Put on your flashers.
Call the Police
Call the police, even for minor accidents. Not every jurisdiction requires a police response to every accident but it's best to report it anyhow.
Exchange information with others involved in the accident. The information you should expect from others and should be prepared to share includes: name, address, phone number, insurance company, policy number, and driver license number for all involved drivers; name and address of all vehicle owners; name, address and phone numbers of witnesses; and license plate numbers of all vehicles.
Keep Control of Yourself
Be polite and factual. When speaking with the other driver and with responding emergency officials, it is important to be polite and to provide required factual information. However, you should NOT accuse others of fault in the accident nor should you volunteer that you are at fault.
Photograph the scene. If you have a camera (perhaps on your phone?) consider photographing the vehicles and the scene. But be careful! Don't wander into traffic or take unnecessary risks while getting your photos.
Make notes. Take note of the make, model, year and color of involved vehicles; the location of the vehicles; and the precise manner in which the accident occurred. If you haven't spoken with witnesses yet, this is a good time to ask for their contact information. Remember, it's alright to ask people for contact info and other information, but don't interfere with the police.
Don't Rush to Leave
Don't leave the scene before the police and the other drivers do unless you must or the police tell you otherwise.
Notify the Insurance Company
Notify your insurance company and/or agent. It's not uncommon for people at the scene of an accident to agree to handle things themselves without involving their insurers. This is often not a good idea. Differences often arise after the fact, particularly when the at-fault driver sees the amount of the repair bill or if there are injuries. You'll likely be better off if you notify your insurer promptly.