The "flange to focal plane" distance for Canon FD mount lenses is 42mm. For EOS mount lenses it's 44mm. That means an FD lens won't be able to come to proper focus without some optics. There are FD lens adapters for EOS mount. I haven't used one, but my guess is that you'll get some degradation of optical quality but it'll "work". You'll have to set the f-stop manually using the aperture ring on the FD lens and, of course, the lenses are manual focus.
35mm film cameras with manual focus lenses had bigger & brighter viewfinders and usually a split-prism focus aid to help you get accurate focus. Since modern DSLRs all auto-focus, and most use APS-C size sensors, the viewfinders no longer need to be so big & bright, nor do they require the focus aids. Consequently, this makes using a manual focus lens a bit tougher than it would have been with a real 35mm film camera.
Since the adapters are usually inexpensive, it's probably not much of a risk to buy one and try it out. I suspect you'll prefer lenses designed for your EOS body.