With a bit of luck someone will have written it on the car's handbook. Otherwise you'll need to contact a main dealer for the make of your car and fork out (probably around) £15 to get the code.
Alternatively you could try an online service, such as this one:
http://www.radio-code.co.uk/
However you need to be aware that online services require you to provide the radio's serial number, which you can only get at by removing the radio from its mount. (Without special tools, that can sometimes be rather tricky). A main dealer can usually provide the code directly from the vehicle's VIN (assuming, of course, that it's the original radio that the manufacturer fitted into the car), so you won't need to remove the radio.