There's a good tutorial for getting SheepShaver running. Just google for it (it contains links to illegal software so I won't link to it here).
Basically, you need the ROM-image from an old mac and the OS.
You can use OS 7.5.5 legally since apple gives it away for free. But downloading a newer version will be illegal if you don't have a license for it (not that anyone will care).
All of the available ROM-images aren't legal either, but I forget which ones. Then again, I don't think anyone will care if you download an illegaly distributed ROM-image, it's not like Apple is making any money off them.