Seed,
If you can sing a melody, you have the potential to sing harmony. To bring that potential out, I recommend Cathy Fink and Marcy Marxer's tape package called "Learn to Sing Harmony."
It is a booklet plus a series of cassette tapes. They do some explanation of what harmony is and then do many examples. They offer opportunities for you to sing along with a part and then to sing it yourself with the melody. They have help from Robin and Linda Williams. They also provide at least two ranges per song. The songs were all familiar to me, all traditional folk songs.
The package is available from Homespun Tapes. The telephone number there is 1-800-33-TAPES. I have found the package well worth the $37.50. It increased my confidence in harmony singing immensely.
Roger in Baltimore