As VHG suggests, the first thing to do is to thoroughly check any documentation which came with the car. (The radio code is often written inside the cover, or on the back of, the handbook for the car or for the audio system).
If you really can't find the code you have a choice of two way of obtaining it. Firstly, you can go to any Ford main dealership (taking your proof of ownership of the vehicle with you). As long as it's still the original radio in the car, the dealer can obtain the code directly from Ford, simply from the registration number (or VIN) of the vehicle. The usual charge for the service is £15.
Alternatively you can use on online service such as this one:
http://www.radio-code.co.uk/
Such services are usually cheaper but they have the disadvantage that they can't provide the code directly from the registration number. You have to provide the serial number of the radio, which means taking it out of the dashboard. (That's frequently a very fiddly task, which either requires special tools or a great deal of patience and luck!).
Chris