Teach Yourself Twinkle, Twinkle, Little Star
Your first song in Chordzy is Twinkle, Twinkle, Little Star, and you play it before you read a note of theory. Music first. The theory comes once your hands have something to attach it to.
You pick the notes out by ear, on your own. If that sounds like a lot, it isn't: the melody is one you have known since you were three, and the game gives feedback without judgment, so a wrong note costs you nothing but another try. Every song in Chordzy has many skill levels. This is the very first song on the very first level, and you can repeat it as often as you like.
Why this song first
Because you already know it. Twinkle, Twinkle, Little Star is catchy and familiar. It is also hiding in two other songs you know.
The "Alphabet Song" (a-b-c-d-... e-f-g-... h--i--j--k--...) and "Baa, Baa, Black Sheep" are actually the same notes.
That is why it works as a first beginner piano song: you find the melody by exploring the keys yourself, and your ear tells you the moment you are right. Nobody has to.
Fun facts
Mozart didn't compose "Twinkle Twinkle, Little Star", whatever the internet says. Still, his name sticks to it because he took the tune and turned it into a set of variations. Not one. Several.
The original melody was a French pastoral song from 1740, and adapted by Mozart into 12 different variations.
So when you pick it out on the piano, you are playing a tune that has been passed from hand to hand for close to three hundred years. It has survived worse players than you.
Using Chordzy to learn by ear
Guess the notes from what you hear in the speakers. That is the whole method.
Sing the melody before you play it, even under your breath. The ear already knows this tune.
Use your right hand to guess, whichever hand you write with. The left hand joins later, and when it does the reason for starting on the right will be obvious.
When we add the left hand, it'll make sense.
Press play and the game begins.
Twinkle, Twinkle, Little Star, by ear. A few notes in the speakers, then the same notes on the keys.