First, get some soft clay, which can be mined. Get a bronze bar. Head over to the jail, located just east of the Draynor Market bank, and speak with Lady Keli. Choose any option except for the one about Katrine being better than her. Ask her about her plan, and then ask her if she can be sure that they won't try to get the Prince out of jail. Ask her to see the key, and then ask her if you can see it. Your character will put some clay in their hand and make an imprint of the key. Take the imprint and bronze bar to Osman nd he'll make a bronze key for you, collect it from Leela.