The Great Deku Tree is desperately ill, he calls for the fairy Navi to go and look for the only child without a fairy in the village. After a rough awakening, our young hero gets up. Leave the house from the left, go down the ladder and speak with your friend Saria.
Visit the Kokiri Village and talk to the children while collecting as many rupees as you can. To do so, lift and toss rocks around, run through the tall grass and open the chests inside Mido's house. Then head to the training area south of the map and go through it. You will find a small entrance carved in the rocky wall. Crawl through this tunnel and on the other side, you will find a chest containing the Kokiri Sword. Equip the sword from your inventory, then crawl back through the tunnel. If you still haven't found 40 rupees, go through the village again, then head for the Kokiri Shop (the house with red decorations over the door) and buy the Deku Shield.
Now go all the way to the east (according to the map) and talk to Mido who will reluctantly let you pass. Attack the three Deku Babas, which will drop Deku Sticks, then keep going towards the tree.
The Deku Tree tells you he's under a curse and that he needs your help. Accept your quest and get inside the tree.