Complete the mission "The Local Flavor" in Point Lookout; it's the first mission you receive. The mission will start right when you arrive at the boat to get to Point Lookout. When you arrive at Point Lookout, you head Southeast to the Calvert Mansion (will be marked on your map) and help the ghoul Desmond defend the mansion against Tribals by exploring the west and east wings of the mansion, respectfully, and sealing off infiltration points. After that, the Tribals will come in numbers through the front door. Kill them and talk to Desmond about the Tribals and he'll ask you to infiltrate the Tribals and find out their aim in attacking the mansion.