Your advert is the only chance you've got to generate interest in your car. With all the other cars that buyers have to choose from, a poorly worded or vague ad will simply be a waste of space. What you do need to include are:
Type of Car
Give the exact make, model and specification of the car, including the engine, trim and bodystyle. For example, don't just say "Vauxhall Astra", put "Vauxhall Astra 1.9 CDTi 16v Design Sport, three-door hatch diesel automatic".
Year
Include the year and registration letter, such as 1999/T-reg or 2003/53-plate.
Mileage
State the exact mileage; don't just say high or low.
Colour
Avoid using the manufacturer's colour name as this will mean little to many buyers. Say whether it's light, mid or dark, then say what the basic hue is, along with whether it's metallic or pearlescent. For example, "dark metallic blue" is a much more helpful description than "Pacific Storm".