How do I get the light arrow ?

I can't be specific, because I haven't done it. You get it after the mini boss in the stone tower. (The 4th dungeon.)

Well, the Light Arrow is located in the Ikana Temple, and you need to defeat one of the mini-bosses (the Gero Master) in the dungeon to obtain it.

I'll explain a portion of the temple, as the beginning is just simple exploration.

As I remember it, you have to have the temple turned upside-down (shoot the red jewel outside). Then go through the right exit (where the blue sun block used to be). You'll be needing a Silver Key to open the locked door ahead in this room.
The next room involves using the Mirror Shield on sun symbols and defending yourself against continuously spawing Black Boes. The objective in this room is to use your Mirror Shield to store sunlight in the left mirror, then run and use this reflected sunlight and store it in another mirror, then use this reflected sunlight to remove a blue sun block.
In the next room, you need to drop down to the bottom area (in Goron form), press a floor switch, then roll to another switch before the flames return. Then return to the door, change to Deku form, and use the vents to make it to the other door (involves a U-turn).
Then you face the Gero Master. He's like a normal Gero, except he does this teleport move that gets you from behind. If he does this, just look for his shadow on the ground, and move away from the place you were originally standing. Then attack him normally. You can also do a thrust attack if you don't mind getting injured.

Kill the gero master in the 4th dungeon in the door with the silver key lock.