You have to have some sort of combat rating, then if you go to the bar of Viking, in the Tichel system, there is a 50/50 chance that you will be approached to make a cargo delivery for Uncle Olaf. If you make do this mission, you have started on the Pirate campaign. Also, you can branch into the Pirate Campaign from the Wild Geese campaign. For Wild Geese, you have to have a pretty high combat rating, and it will appear in the bar of Earth, Sol System as taking a man to a wedding.