Get to a doctor
If you have a severe injury, obviously you need an ambulance to take you to the hospital. Even if your injury is not severe, however, you should go to a doctor to check. Some injuries worsen with time and can become worse without treatment. The worst that can happen is that the doctor will look you over and tell you that you are fine. Your health is the first and most important consideration.
Call the police
Even if it seems like a minor accident, you should call the police. It is not unheard of for the other driver to deny that an accident ever happened. The police can document what happened and get complete information from the other driver. The police will also conduct an investigation and document it. This may be helpful if you ever need to file a lawsuit.
Don't argue with the other driver
Even though your first inclination may be to yell at the jerk who crashed into your car - don't do it. If you are able, inquire if anyone was injured. Call for help. Exchange insurance information. Anything further that you say may only serve to start an argument, and may be used against you in court. The police are the only people at the scene who have a right to ask you about the collision.
Don't move the car
If your car is in a dangerous location after the accident, you should move it or move away from the car. If it is not in a dangerous location, leave it in place until the police arrive. Police officers can tell a great deal about how the accident occurred based upon the locations of the cars and the physical evidence. If you move the car, they will lose that opportunity.
After you have attended to your injuries, it is a good idea to take photographs of your car - before it is repaired. If it is possible to get pictures of the collision scene and the other car, they can be helpful as well. Finally, if you sustained an injury, take pictures. Over time, things change and memories fade. Physical evidence can become critical. It may sound overly cautious, but the other driver's insurance company will absolutely work to preserve evidence that helps their case. You need to do the same.
Don't discuss the case with anyone
The other driver's insurance company is likely to call you after the collision and ask what happened. You may not realize that they are recording the conversation. Their questions may be unfair or misleading. What you say can be used against you later. Sometimes the person who caused the collision will call. Discussing the case with them carries the same risks.
Consult with a lawyer
If you go to the doctor and he gives you a clean bill of health - that's great. Even if you have a minor injury, you may be able to resolve your dispute with the other driver very easily. A good lawyer will listen to your situation and tell you if you do not need to hire them. Unfortunately, there are insurance companies that deliberately give you the run around. The worse your injury is, the more likely they are to fight your claim. In cases involving serious injuries, legal help can be critical.