One way or the other, make sure you're now adult Link and head for Kakariko Village. Go up to the Death Mountain Trail and travel to Goron City. All the Goron's are gone except one rolling around on the third level. Use a bomb to stop him from rolling and talk to him. He'll give you the Goron's Tunic, which is resistant to fire. Equip it. Now go to the bottom floor and enter the King's room. Pull the statue at the back of his room out of the way and enter Death Mountain Crater. Use your hookshot to get across the broken bridge and on your way across it Sheik will come and teach you the Bolero of Fire. Now continue on down and enter the Fire Temple.