On World One, Level One, get a red mushroom, and, if possible, a leaf (Just in case). Jump on top of the white platform. Hold down the down button until you drop behind the white platform. Then run, and don't stop even if you run into an enemy. You can jump if you need to. When you come to the black area, you will go behind it. You will go into a room, where you will see toad, and he will give you the magic whistle. This allows you to go to any of the next 3 worlds.