I'm answering this older question because, in my experience, it may help a current reader. Hope nobody minds.
Virtually anyone (except maybe someone who’s tone-deaf) can easily hum, sing, or whistle their favorite tunes. You don't need any specific musical training to do THAT.
With the right teacher who can already play by ear, it's just a matter of assisting you in learning how to transfer the music you can already hum, sing, or whistle, to the instrument you wish to play. And of course, once you've been taught how to do this, you will need to invest some time and effort in polishing your new skill.
And, you don't necessarily need great musical talent to learn to play by ear. Anyone, with the right teacher, can learn to play any instrument by ear, consistent with the amount of inner musicality with which you were born. Everyone is born with some degree of inner musicality (again, except maybe the tone-deaf).
Furthermore, unlike traditional music instruction, learning to play by ear doesn't require a lot of tedious drilling and boring technical exercises. That's good news for anyone who just wants to have fun with music— and doesn't need to become a tremendously accomplished musical artist.
Although any instrument can be played by ear, I emphasize learning to play the keyboard by ear—which is my speciality.
With the keyboard, using two hands, you can learn to play the melody with the right hand, and the chords (harmony) with the left hand. Although this may sound like a coordination challenge to some people, it really isn't that challenging. While you’re playing the melody with your right hand, you’re just holding down a few notes composing the harmony to that melody with your left hand. With some practice, you can master this skill.
This info came from my more detailed YouTube videos—and accompanying free ebook—if you're interested. Media URL: http://tinyurl.com/My-4-YouTube-Videos
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