Am I an Alto, Mezzo or Soprano? (re-posted with a different video)?

I'm joining a choir nearby in the summer so I can get volunteer hours
for school, but I've never been in a choir before so I don't know what
part I should sing. There is no audition and no solo singing, anybody
can join at any level of singing. Can someone listen to my singing in
the following video and tell me which part I should sing? I know I'm
not very good, but I'll be singing in the background so no one will
notice :) I'm also kind of nervous in the video since I've never made
a video before.
(I re-did the question because the song I picked didn't have much
range, hopefully this will be better, and I know the choir did this
last year.) Most of the songs I listen to are metal, so I don't know
many choir-y songs off by heart, that's why I'm singing this the way
the choir did before and not how to original is.

http://www.youtube.com/watch?v=h20iovGM2kk

There is alto and soprano for sure, they only add the mezzo part if
there is enough singers