Traditionally, people who learn the piano also learn to read sheet music as this is a convenient way to learn how to play songs. But now and then someone by-passes learning to read music in favor of playing by ear. Usually these tend to be self-taught pianists.
Playing the piano “by ear” really just means playing without the aid of written music notes. If you have a really keen musical ear, you might find it pretty easy to match the notes on the piano to what you hear on the recording of the song.
Once in a while, someone has this gift in enormity and can effortlessly pick out a well-known tune on the piano without making mistakes, or at least, very few.
When children learn the piano, including learning to read the notes, they are usually encouraged to develop their “ear training”, which means to recognize the interval between two notes. Recognizing if a tune is moving up or down is a good skill to have and this is where to start if you want to play by ear.
Sit at your piano and play the song you want to learn on your device. Just play the first line or two several times. Now find the first note of the song – you might want to practice singing it along with the recording to begin with – then find that note on the piano. Play the note that’s your first best guess, and if it’s not an exact match, figure out if you need to go higher or lower. Keep hitting notes until you hit the note that matches what you’re hearing or singing.
Now find the second note of the song. Is it higher or lower than the first note, and by how much? Again, play some notes until you get a match, and then move on to the third note. Depending on how advanced your musical ear is, you might find this goes quite quickly, or it might be a slow process. Stick with it until you have the first line of the song picked out. Then play it over several times until you’ve memorized it. You’ll probably find that the rhythm will come to you more easily than the tune.
Once you have the first line or two, you might find it gets a bit easier. The more you do this, the quicker you will get at picking out tunes.
If you want to accompany the tune, you can play the tune in your right hand and play chords with your left hand. You can work out which chords will go where by ear, but you do need to know the chords first. There are many piano lessons and also free videos and articles that teach how to create chords. In the meantime, you could try just putting one note in the bass with the left hand. Use your ear again to hear which note sounds good.