I have my daughter's abandoned Galaxy S in front of me now. (She's moved on to S2)
Settings - Sound - Phone ringtone - allows you to change to one of the ringtones listed.
Music - album - track - touch bottom left thingy - Set as - chose phone ringtone.
So if that's how you do it, whatever the sound that you have is saved as, you need to import it in to the phone as something that the phone will see as music. I've found Samsung Kies to be OK, but it will only work effectively if I sign in to the pc as administrator. As the man from bracknell, mixture et al tell us, I only do that when I have to.
If you want, you can import music direct to the phone, using the phone as a drive. However, if you do that, Kies sometimes won't see mine until I demount the micro sd thingy, which is a pain..
I've also used a recording that I recorded onto the phone, (voice dictation) but I can't remember how I did that.
BOAC has a samsung android, and will, no doubt, be along shortly with a more elegant solution.