Death Mountain Crater:
Head to Death Mountain Crater. You'll need the Goron Tunic (bought in Goron City or found in Darunia's room) to survive the heat. Play Bolero of Fire (learned from Sheik) to warp directly to the crater. Enter the Fire Temple.
Fire Temple:
This dungeon involves rescuing Gorons and navigating lava pits. Obtain the Megaton Hammer. Use the Hammer to hit rusty switches and break cracked blocks. The boss is Volvagia. Hit him with the Megaton Hammer when he emerges from the lava, then attack with your sword. After defeating him, Darunia awakens as the Sage of Fire, granting you the Fire Medallion.