Where do you need help? You can check the walkthrough of the site at the top of the page.
The first step of the dungeon is to float to the bottom and go into the east (right from the start) passageway. You'll see Ruto there and can lower the water for the first time above where you meet her.
Explore the chambers around you to find things; you need to check things every time you change the water level to figure it out.
There's also a trick that makes the dungeon a ton easier. There are six small keys but you only need five to beat the dungeon because you can bypass a door. Once you lower the water to the bottom, you notice that you can now enter the center chamber which has a locked door. However, I would suggest that you play the Serenade of Water, re-enter the dungeon, and jump down to the second floor of the center chamber. If you walk around the 2nd floor of the center column, you'll see a barred door with an unlit torch next to it. You can light the torch with din's fire or by shooting an arrow through the lit torch nearby. Doing this let's you bypass the locked door on the ground floor so you can miss any key in the dungeon and still finish it.
You should be able to find two keys after lowering the water to the bottom, two more after raising it to the middle, one after raising it back to the top, and one more at the middle level after finding the dungeon item. Since you can skip the last one if you follow my advice, you should be able to pass the dungeon with changing the water level three times.