My two-cents worth, from a guy who wishes he'd done things bit differently . . .
"Learning to play" is a deceptive phrase, because it suggests that learning an instrument is solely about "producing" musical ideas. It is equally important to "receive" musical ideas and to develop your internal aural map of the musical landscape. The real magic happens when your musical mind and your skills as a player combine to produce something new. Thus, ear training should be undertaken on the same day you decide to learn an instrument.
Ear training? At its most basic, you must learn to distinguish between musical intervals in a given context. Let me give you an example. A major triad is defined as the 1st, 3d, and 5th intervals of a scale played in unison. C major, for example, is defined as C, E, and G played together. When you strum this chord on the guitar, those essential notes are in there, along with a couple of redundancies. Fiddle around with them, and the chord changes quality - sometimes for the good, sometimes not.
Train your ear to hear these intervals in pairs. Play the C and then the E. Sounds kinda incomplete, doesn't it? That's the 3d interval. Now play C and the G. Still incomplete, but we're getting close. That's the 5th. Those pairs each have a different colour. Blend the two and you get C major. Now listen to the Dixie Chicks, or some vocal group that sings nice harmonies. Hey! You know how that works. Each singer is singing a different interval and together, they're singing . . . a chord!
Practice your ear training every day. Use your instrument, or find some software to help you. Functional Ear Trainer is great. And it's free. Just do it. I guarantee that, unless you have a tin ear, you'll find the music you hear and play will make a lot more sense to you at an intuitive level.
