Enter goron City and use bombs to stop the small fast rolling Goron. Talk to him couple of times until he hands you the red tunic and opens up the doors around the place. Now go into Big Goron's room and push the big statues. This opens up an entrance to Death Mountain Crator, Equip the tunic and go on in.

Work to the end of death mountain crator and go down the ladder to arrive in the Fire temple.