How do I assign a ringtone to a contact with my Samsung Galaxy Mini?

I am at a loss as to how to assign a ringtone to someone in my phonebook? If you click 'edit' on the person in the phonebook on the Galaxy Mini, it only gives you the option to edit the name and number, but not assign a ringtone. Similarly, if you click on the ringtone itself, it only gives you the option to assign a ringtone to the whole phone, but not an individual. Help please!