In order to begin life as a Marauder as a secondary job, the first step is to accept a quest from Blauthota in the Coral Tower. Secondary jobs typically have you talk with the Guild Receptionist, who gives you a spiel about the great history of the Guild. After that they ask if you want to join up. When you say yes, you get to go speak to Wyrnzoen and receive the Weathered War Axe. Equip the axe (which typically leaves you in your underclothes), finish the quest and then begin the typical guild quests.

Have fun with it.