This song is taught to young Link by Malon. Malon and Talon will not have returned to Lon Lon Ranch until after you have met up with Princess Zelda for the first time in Hyrule Castle. Travel to Lon Lon Ranch afterwards and speak to Malon in the center of the horse pen. Speak to her twice and she will teach you Epona's Song.
Epona's Song is used later on in the game as adult Link to call over Epona. This is part of the side quest to obtain Epona the Horse - click on the link for more information.