In the beginning, you can do small side-quests, defeat opponents and sell items that drop. Later in the game, this unlocks new ways to make gold, such as training and selling slaves, and the Ardass guild.
Thanks for the fast reply! but how do I up my rank in the guild?
For the time being, this is not yet possible. I need to expand quests and guild ranks. Sorry... xP
Ah no worries, I am loving the game so far and am excited to see what the future may bring.