How do you increase your magic bar in Zelda ocarina of time?

Answer 1

You can visit one of the Great Fairy fountains and talk to the great fairy. She will increase your magic bar and heal you or you can cut grass and collect the bottles of green potion. :)

Edit: The Great Fairy that doubles your magic power can be found inside of the Death Mountain Crater. You will need to use the Hammer to break down the stones guarding the entrance.

Answer 2

you have to find another fairy like the first you one you saw and once you find one they increase your magic power

Answer 3

Visit the fairy in death mountain crater. She's in a cave blocked by rocks on the opposite side of the entrance to the fire temple