by Alycs » 03 Aug 2013 14:31
I opened up the vocals for the song on Newtone and it appears that it is in E Major. (I could also be in A, as the song has no D's or D#'s to distinguish, but both starts and resolves on E, so its a safe bet)
Hope this helped.
As for practice, I don't know what instrument you are playing, but a general rule to follow is to get the melody down first, and then (if you are playing a two handed instrument) find the basic chord progression and just play that along with the melody until you feel comfortable, then start adding little touches.
If you can't figure out the melody, try downloading one of the accappella's and opening it in something like Melodyne, Newtone, or Ableton's midi view.