Begin the information gathering process.
My standard advice is to begin the information collection process. You're going to want to collect information, certainly, about your own physical well-being, and the well-being of anybody else that's in your car. Once you've secured that, once you know you're okay and you're able to make some decisions, you want to start to collect as much information as possible at the scene, information that might be lost later. You're going to want to know what road you're on when the accident happened, what the intersecting streets were. You're going to want to know identifying information about the other driver, you're going to want to know what their driver's license number is, you're going to want to know what the tags on their car are, you're going to want to know what their insurance information is. You should be particularly careful to note the presence of any cameras, whether municipal or private cameras that might have captured the event. Certainly, pictures, as the old adage says, are worth 1,000 words and you should absolutely take as many pictures as possible.