Press the green flag and wait for a while. When the piano pops up, you can click on the keys with your mouse, or press the keys on your keyboard. For example, you press 'a' to play the low C key, while you can press 'q' to play the low C#/Db
Thank you for all the people who decide to play the piano. I will be doing an update soon that will let you learn how to play different songs. Ideas for songs in the comments will be accepted.