1 am not sure what this question means because I learnt piano as a kid, and you don't learn in any "particular key"....as Vertivert has pointed out, you first have to learn to read both treble and bass clefs. I guess most of the early simple tunes and exercises you learn as a beginner are in C major because it doesnt have a key signature to remember, but you soon learn what other keys mean, and that they have different key signatures that affect the way you read the notes on each clef ...some of these I found more difficult to use than others!
It takes time to become accustomed to using each key. As a kid I also taught myself to play the recorder. I had both a descant and a tenor recorder; these are both based in C major and that is the easiest key to play in, until you learn to form all the various sharps and flats.