Start with the key of C and three notes. Once you get used to it, you can change the key, increase the number of notes, or turn the keyboard sideways to widen the range. You can also play melodies you know freely. Even if you cant get the notes right, just listen to the root note and then hum it, or listen to it repeatedly, and youll gradually be able to get the notes right. If you practice every day, youll be able to play difficult keys smoothly.