Saria's Song is the second song learned in Ocarina of Time that you are required to learn to progress. This song is used to talk to Saria or to Navi, but this usually isn't necessary. It is also used to talk to Darunia. If you play it in front of him when you first meet him, he will stop being so grumpy and dance. He will then talk to you.
Where to learn it
After you have met Princess Zelda and have learned Zelda's Lullaby, you are required to go back and tell Saria about the quest you are on. Go back to Kokiri Forest and go into the Lost Woods. If you walk up to each of the entrances, through one of them in each room you will hear this song get louder. This is the entrance you are required to go to in order to reach the Sacred Forest Meadow. Once you reach the Sacred Forest Meadow, you just go through the corridors until you reach an area where Saria is sitting on a tree stump. Talk to her, and she will teach you this song.
How to play it
In the original Ocarina of Time, it was played using ↓ → ← ↓ → ←.
In Ocarina of Time 3D, however, it is played using R Y X R Y X.