Tranquility

Head to the Hanged Man in Lowtown and talk to Varric. He'll let you know that you'll need maps to the Deep Roads and to seek out a Grey Warden. Head to the Ferelden imports shop in Lowtown and ask the merchant about the Grey Warden Healer. Afterwards go to Darktown and into the clinic to meet with Anders. He will agree to give you maps if you help save his friend Karl.

Head to the Chantry at night and you'll find that Karl has already been made Tranquil, selling you out to the Templars. Fight the incoming Templars, focusing on holding threat against the Lieutenant and Hunters wile you clear out the lower end Archers. After the smaller enemies are dealt with, have the whole party gang up on the Lieutenant. You'll probably be too low of a level at this point to have an array of skills, but if you have access to Stagger/Brittle/Disorientate, use a class-combo attack to deal extra damage.

After killing the guards, convince Anders to kill Karl to gain some Friendship. You'll be taken back to Darktown where you can further help or hurt your relationship with your new party member. Since your business is done at the Hanged man, returning there will now start the Side Quest Fools Rush In.