I tried to click it on the usual way but it didn't work. Do I need to buy a specific lens mount? The person who sold me the lens said it worked with his Canon A-1 but perhaps he had a lens mount with it.
There are two versions of the Canon FD lens mount. Both versions will work on your camera, but they attach and lock differently. With the original FD mount (which this lens may be), there is a ring at the back of the lens, behind the aperture ring, that needs to be turned in order to lock the lens onto the camera body. With the "New FD" or "FDn" mount (well, in 1979 it was new) you turn the whole lens (like you do with most lens mounts), which means that you need fewer hands to juggle lenses as you change them, and it's a lot easier to work with gloves.