How much is the car’s book value and how much are you willing to accept? Check the car’s value on Kelly Blue Book (kbb.com) for a “Private Party Sale” in your zipcode. Edmunds.com is also another site to check. Those values are the most buyer’s would likely be willing to pay.
Write down facts and good things about the car and about your ownership of it as a basis for your ad. Important facts about the car include the vehicle’s year, make, model, mileage, transmission time (auto or manual), air conditioning, how many airbags, anti-lock brakes, engine size/type, color, number of doors, whether it’s 2 wheel drive or 4 wheel drive, what options it has (navigation system, DVD player, CD, MP3, etc).
Facts about your ownership of the vehicle include whether you’re the original owner or how long you have owned it, other states you have lived with this car, if there are complete maintenance records, if it’s been in any accidents, if it is still under warranty and so forth.
Other good things to include are if you’re a non-smoker, don’t drive your pets around in it (dog hair is hard to remove from upholstery). You might also want to include your opinions on what the vehicle is good for.
For example, in the case of an SUV it could be useful for hauling things, going places in snow, or off-roading and so on. Mention things you have used it for that a potential buyer would also likely find useful.
Once you’ve done these steps I’d suggest advertising it on CraigsList with the photos you took, possibly in the local newspaper and other places online (Facebook marketplace is growing). Don’t put your phone number in the ad. Let online prospects contact you by email before proceeding in a way that you’re comfortable with (though if you advertise in the paper you’ll need to list your number). That way you’ll also have an easier time managing the information.