Kokiri Forest:
Start by talking to Saria and Mido. Mido won't let you pass without a sword and shield. Obtain the Kokiri Sword from the training area (crawl through the hole) and the Deku Shield from the shop (40 Rupees). Once equipped, Mido will let you see the Great Deku Tree.
Inside the Deku Tree:
Navigate the dungeon, defeating Gohma Larva and using Deku Nuts to stun enemies. Collect the Fairy Slingshot. Your goal is to reach the bottom floor and defeat the boss. Remember to burn webs with Deku Sticks and use the Slingshot on switches.