Click on "my account" in the upper left corner of the Craigslist home page. Then click "sign up for an account" at the bottom of the next page. Follow the instructions. Once your account is set up, click on the "post to classifieds" link (immediately above the "my account" link).
You probably want to post your ad in "for sale" --> "cars & trucks - by owner". Take a look at what similar cars in your area sell for or check the Blue Book to set your price. Be sure to include the information buyers want to know (year, make, model, mileage, condition) and a few pictures of the car.
I sold a car off Craigslist a few weeks back. It only took a total of maybe 5-10 minutes to put up the ad (and I write/think pretty slow).