You'll received this quest after your first encounter with Varric. Just after you finish the The Destruction of Lothering quest. Your task is to talk to Aveline who has been working as a city guard for the past year.

First find your way into Hightown. From there, Go to the most southern part of map. Note that you can only get inside the Keep during the day. You'll find her standing by the corridor of the City Guard Barracks on the southern part of the Keep.  Here you'll discuss her life as a city guard. Eventually she'll offer you a job. Its up to you to take the offer or let the guards do their jobs. Accepting it will unlocked a  new Companions Quest. Either way, she'll join your party and that will complete the quest. Additionally, You'll receive a Aveline Vallen codex entry.

If this is your first time in the Keep you'll find several codex entries, a locked chest and an additional sidequest. (1) Heirarchy of the Circle - codex entry, (2) The Llomerryn Accord - codex entry, (3) The Enigma of Kirkwall - codex entry, (4) Bottled Scar 5:34 Exalted - sidequest, (5) Locked Chest - Standard, (6) Chantry Hierarchy - codex entry, (7) The Kirkwall City Guard - codex entry, (8) A Study of the Fifth Blight, Vol  1- codex entry, (9) Locked Chest - Simple

COMPANIONS QUEST UNLOCKED : The Way It Should Be.