Check to see if any body parts are falling off.
No? You and your passengers are still in one piece? If you're not hurt, try to assess the scene.
How's your car?
Flip on your hazards (hint: look on the dashboard for a button with a triangle on it). If you're safe from oncoming traffic, turn off your car and get out. If your car is drivable, move it off to the side of the road.
Call the cops!
Painful as this seems, calling the police is the best thing to do. Depending on where you are, they may not come to the scene to check things out. But at least you've reported the accident.
Call the 'rents.
Seriously, they may wig out about the car later, but right now they just want to know that you're okay. Plus, they can come and pick you up if you need a ride.
Call your insurance company.
The sooner they know what happened, the sooner they can help you out. If you have your insurance card handy, that will be very helpful. Your mom and dad should have the necessary info if you don't.