"You got a Deku Shield! Switch to the Equipment Subscreen and select the shield. Press (A) to equip it. Press (R) to crouch and defend. If you press (R) while (Z) Targeting, you can move while defending."
The Deku Shield is the first Shield Link acquires in Ocarina of Time. It is required in order to see the Great Deku Tree, along with the Kokiri Sword, because Mido will not let Link pass unless he is properly equipped. It is child sized, therefore can only be used as a child. It costs 40 Rupees at the Kokiri Shop and 50 Rupees from Business Scrubs.
The Deku Shield is made of tough, hard wood, and it gives great protection. Unfortunately, because it is made of wood, it burns instantly when it comes in contact with fire.