...and notify the authorities first. If it's a fender bender and you're on the freeway, it's usually OK to move the vehicles to the side of the road before the authorities arrive.
Give the other driver your name, address, driver's license number and insurance information. Make sure you get their information, too. If the other driver's name doesn't match the name of the insured, find out what their relationship is, and make note of both of their names and addresses.
Don't discuss the events of the crash, or admit fault at the scene. Many people are so shaken up after an accident, they immediately think they did something to cause it. Until all versions of the story are known, you can't even be sure you were at fault, so it's best just to not say anything.
Take pictures of the damage and make a written documentation of the accident. If there are witnesses, get their names, addresses, and telephone numbers.
You may not be able to do this at the scene if the crash was minor and there were no significant injuries. You still should go to the local police station or Department of Motor Vehicles website to report the accident.
It's important to plan ahead for car accidents, even though (by definition) you don't plan on having one. Knowing what your car insurance covers and being aware of your deductible will help you be prepared in the event of a car accident.