After you receive the Fire Medallion, your next stop will be to Zora's Domain. There is a major problem here, though. Everything is frozen, including the King! The answer to your puzzlement lies in Zora's Fountain. When you arrive there, you will see many floating icebergs. Hop across these icebergs and you will enter the Ice Cavern. This is really not a difficult stage. You should soon stumble across a torch with blue flame. You can "catch" the flame in an empty bottle. The more empty bottles that you have, the better. Fill them up with the blue flame. Then carry them around to the treasure chests that have frozen over and open the bottles on these.
The flame should melt the ice and allow you access to these treasure chests. There is also some ice that is blocking a doorway - the blue flame works here, too. Continue in this fashion until you get the map. Use the map as your guide to explore the rest of the stage. You should eventually come to a very large room with rupees on pedestals. In this room, there is a rather large block of ice that you can push around. Use this to get at the rupees. Unfortunately, the ice block will keep sliding until it hits something (such as a pedestal) or else it might fall into one of the chasms. You must study the room very carefully to be able to collect all of the rupees. After you collect the ones on the pedestals, you might have to push the block underneath one of the elevated openings in the wall - for there are some things there as well. Once you collect all of the rupees, a locked door should open up. You need the sliding ice block to reach this door. After you go through the door, continue until you fight a large wolf. After the fight, a large treasure chest will appear. This contains the Iron Boots, which allow you to walk on the bottom of lakes and other water-logged areas. Around this time you will meet Sheik who will teach you the Water Rondo.
To exit the level, you must put on the boots and fall into the small opening in the ground. You have to walk under the water for a bit, and then you will end up near the beginning of the Ice Cavern again. However, do not leave the Ice Cavern just yet. Before you go, collect one more bottle-full of blue flame. Then leave the cavern, and open the bottle on King Zora. He will be thawed out and will give you the Blue Tunic. This "armor" somehow allows you to breathe while you are walking underwater (you may have noticed before when you were walking underwater that you only had a limited time or else you would choke). Now play the Water Rondo. You will be warped to Lake Hylia, if "lake" it could be called, for almost all of the water is drained out. The entrance to the Water Temple is locked at first, I believe. You must strike a switch on top of the temple to open up the entrance. Then you must put on the Iron Boots and submerge yourself and then enter the temple through the door. The Water Temple itself is covered in the next section.
Proceed to Water Temple...