On garageband of a macbook, how do i harmonize my voice?

I just bought a macbook pro (i love it!!!!) mainly to work on my music. i post covers on youtube and with my new macbook, i'd like to be able to edit my videos so that i can sing two parts, like a duet, by myself. how do i do this?

here's an example of what i want to do: http://www.youtube.com/watch?v=pT_klCRWzZU&feature=channel
(just the harmonizing part, where she overlaps her voice)

thank you!! i hope this made sense haha :)