Call the police and report the accident, even if the other driver admits fault on the scene and asks you not to call.
Take pictures of the damage to all vehicles, and if you can do so safely, the scene of the collision.
If it's safe to do so and there are no life-threatening injuries, move the cars out of traffic and over to the shoulder.
Get the other driver's license information, insurance information, and car registration information.
If you are hurting anywhere, go to a doctor or hospital. Once there, explain all the pain and symptoms you are experiencing, no matter how minor. Symptoms often get worse in the 24-48 hours following a collision.
Contact an attorney to protect your rights and fully document your claim.