Prior to Hyrule Castle
You hear Zelda plea for help. However your uncle is going. Instead of listening, get up and go out the door. You can either grab the lamp here or in the castle. Go outside and go to the right road. Follow it up, and then go left. One at Hyrule Bridge, go up. Go right at the guard, until you can pick up a piece of grass, that is really a secret entrance.