When you take control of Link you are inside of his house so go outside and you will see Saria coming towards you. Go down the ladder and talk to her after you talk to her go left across the water and talk to Mido and he will tell you that you need a sword and shield to talk with the Deku Tree. Next go to the Training Area and go to the very back and then crawl through the space. After you come out go to the very back see map at bottom. And you should be looking at a treasure chest open it and it has the Kokiri sword. Now make your way back to Kokiri Forest once there go into Mido's house and open the treasure chests. After You get 40 rupees go to the shop and purchase the Deku Shield. After you buy the shield and buy the sword equip them both and talk to Mido. He will let you pass so go up to the Deku Tree and then there will be a cutscene after the scene walk into the Deku Tree's Mouth.