After you get it, tap down to crouch, then tap down again to turn into it. You can move back and forth with it, but you can't jump with it unless you get the Spring Ball, a hidden item later in the game. You never need the Spring Ball, but it can save you time.

If you don't know how to use the morphing ball, you probably shouldn't be playing this game. This is by far the easiest, most simple part of the game. Why don't you try Tetris or Super Mario. Maybe your sister can help you.