The Punishment for Treason (20 points) - You have found the traitor and have brought him before Al Mualim.
You get this achievement after you finish your first mission. Once the mission starts just into the town of Masyaf and do each task (eavesdrop, pick pocket, and interrogate. This first mission is simple and is more like an in-game informational tutorial for eavesdropping, pick pocketing, and interrogations. You should have no problems getting it.