I have been doing electrical work in the US for almost 5 years both residential and commercial. What you will Need to do is remove the light from the ceiling and check to see what wires are inside this box. You should have as a minimum 2 wires, a neutral(white or natural grey in the US)and a hot, both hooked onto your light fixture. What you will be looking for is another wire that is neither of the ones tied to the current fixture. With a small volt meter you can determine if this wire is a constant hot or is switched as well. If the wire is not switched then you should next check to see if the box is a standard light box or if it is a fan rated box. If it is not a fan box, you can purchase a remodel fan box designed to go in the existing hole once that box is removed. Make sure to mark which wired are tapped on to which, as that can be the devil to figure out if not properly trained. When you get your fan make sure it either has a light kit, or you seperately purchase one to install. Where the wires come out of the fan you should have a black, white, blue/red, and a green or bare wire. Hook the black to the constant hot, and the blue to the Switch leg. Tie the white to the neutral and hang fan. Since you will be controlling just the light with the switch it would be a good idea to purchase either a fan with a remote, or some chain extensions. Its always easier to do it right the first time.
