After meeting Zelda and getting a letter from her, you can meet Saria in the Lost Woods and learn Saria's Song from her.
Go to the Lost Woods
In Kokiri Forest, climb up the ledge behind Mido's House, which is the first house on the left when you enter the Kokiri Forest, and then climb up the vines on the wall nearby. Go up the slope to find a hole in the wall. Go through the hole in the wall to reach the Lost Woods.
Get a Bigger Bullet Bag
To find your way through the Lost Woods, approach the holes and listen to the music. When the music gets louder, it means that you are standing next to the correct hole. In the N64 version, you might also be able to tell which way is the right way by looking at the hole. If the hole has a spot of light inside of it, that means that it is the wrong way and it will warp you out of the Lost Woods. If the hole is a flat black color with no spot of light, it takes you deeper into the forest. This does not work in Ocarina of Time 3D.
If you want to quickly reach the destination and skip the upgrades that you can find here, then starting at the entrance to the Lost Woods, go right, left, right, left, straight, left, right.
If you want those upgrades and items, first go right. Then, straight ahead there is a big drum hanging from the tree. Aim at the center and get three bulls-eyes in a row (they are 100 points each) to get the biggest bullet bag. In the 3DS version, if you are having trouble aiming with the motion controls, you can disable motion controls by pressing Start then tapping Options to turn off Motion Controls.
Get a Piece of Heart
Then, climb down to the area below and walk onto the log that has a circle on it. Skull Kids will appear on the log across the way. Take out your Ocarina and agree to play. Win three times, first for one rupee, then for five rupees, and third for a piece of heart. My strategy for this is to play the first few notes quickly, not on the beat, because this gives you extra time if you don't remember the following notes right away. Now climb up the ladder and take a right.
Reach the Sacred Forest Meadow and Learn Saria's Song
After the owl talks to you, go right, left, straight, left, right.
Walk into the meadow. A wolfos appears. To kill it, let it come near you and then swing your sword, but don't let it stay close to you for too long or it will swing its claws. After you kill it, the bars will go down. Walk to the place where the bars had been.
Go to the right. You will encounter Mad Scrubs along this path. Raise your shield to make the nuts bounce back at them. After a nut hits it, run up to it and hit it with your sword to kill it.
One of the patches of tall grass goes straight through the wall. Go through it and go to the right. Continue along the path and you will have to jump into some water. Swim across to where there is another ledge to climb up. From there, turn right when you can, to go up some stairs. Kill the two Mad Scrubs up here, then go up the stairs and watch a cut scene. Agree to play the Ocarina with Saria to learn Saria's Song.
Get a Piece of Heart
To get out of the Lost Woods quickly, you can save and reset to go back to your house in the Kokiri Woods.
Go to the entrance of the Lost Woods and turn left from the entrance. Stand on the short stump and play Saria's Song to make the Skull Kid give you a piece of heart.
Go to Kakariko Village
Your next destination is Kakariko Village. The entrance to it is in the northeast of Hyrule Field, east of Hyrule Castle.