The whistle is a diatonic instrument with simple fingering, and it's very easy to learn to play - easier than the recorder. If you're thinking about beginning the whistle, it is advisable to begin with a normal (high) whistle in the key of D. It is generally considered to be the easiest to play, and pretty much all of the tutor books you can buy are for whistles in D (although they can be used for any key).