Talk to Jex, who is in the North-east part of the city in a building with a broken altar. He will tell you how to access the dungeons beneath the building. Go down the ladder east of Jex and talk to the guard if you haven't got any lighting source already. He will sell you a torch for 200 coins, and a tinderbox for 50 coins. A bullseye lantern is a better choice because it will go out less often. Alternatively you can take a Seers' headband, or the silver and gold torches from the Gielinor Games, as these light sources never go out. It is strongly recommended that you take good food (monkfish or above), an emergency escape teleport such as the ectophial, a tinderbox, and an antipoison potion or two.