The first available case is the takedown of Popstar and comprises three mini-missions. The first mission involves beating up some deadbeats for Popstar, then escaping the cops with the package. The second mission tracks down Popstar's supplier and introduces you to the drug bust diversion, which will help getting many of the gold stat medals.
The final missions is not available until completing the Mini Bus Racket mission, which should be the third main story mission. Tail Popstar, take pictures of the deal, fast talk your way into the docks, then continue to take pictures. When you leave, be ready to flee from the cops again, though it seems particularly easier this time around. Once you park the van back at the undercover HQ, the mission ends and the achievement pops.