What letter is the first key on the piano?

I learn songs from this youtube channel http://www.youtube.com/user/musicblueprints and the first note I see on the piano is a "C" I heard from people it's a "A" but I don't really know which one is first, and if it's a "A" then why does that youtube channel's videos piano have a "C" in the beginning and not a "A"?