One way is to take your car to your local car salesman and ask whether they can do valuations. Depending on the age and condition of your car you may be able to get a part exchange on a new car. There are many finance options available these days if you can't afford to pay out all at once.
Valuations take into account various information already obtainable from your car such as: Make, age, mileage, damage, previous owners, remaining MOT and servicing history.If you own an old car you can advertise it on the net, in the local paper or simply by putting a 'For Sale' sign in the car window.