Kakariko Village
Now that you're in Kakariko Village, go up the stairs to the left of the main
path, and then go up the next set of stairs near the lookout tower. Go up to
the guard by the gate and show him Zelda's Letter. He'll talk to you and
he'll eventually tell you to go to the Market and buy the Hylian Shield.
You'll get a discount on it after talking to him! While you're in the Market,
stop by in the now open Happy Mask Shop and borrow the Keaton Mask. Now that
you have the mask and the shield, go to the Lost Woods by entering it from
Kokiri Forest.