To defeat the first Boss, go to the top right of the room and step on the switch. Then, go to the bottom left and step on that switch. Next, go to the top left and step on that switch. Finally, go to the bottom right and step on that switch.
Extra World "Temple of the Frog"
Once you beat all of the normal four worlds (Goblin Caverns, Ancient Ruins, Sea Town, and Sky City), you will find the final 5th world, Temple of the Frog, on the map. However, to enter this world, you must have collected at least fifty coins throughout the game. Once you do that, you can enter Temple of the Frog, and face the final boss of the game: Mr. D.