Once your character has the skills required
Pick up a craft. Synthesizing items, woodworking, blacksmithing, and alchemy are all examples of crafts and skills in Mortal online gold that pay very well once your character has the skills required.
Go questing. Although this has a relatively minor cash incentive, most missions and quests will at least give you some form of compensation, be it cash in hand, or a rare and valuable item that can be hawked at the AH to another player.