The Ancient Cistern is the first dungeon in which Link is tasked to find one of the three Sacred Flames. In the Ancient Cistern Link acquires the Whip which allows him to progress deeper into the dungeon. After defeating Kolotos, boss of the Ancient Cistern, Link gets the first Sacred Flame.
Ancient Cistern Walkthrough
Ancient Cistern – Acquire Whip
Finishing the Ancient Cistern
Kolotos Boss Fight