Start by going to Rorina's castle south of Thais, here:
First Part
Ask Rorina for a mission.
Player: hi
Rorina: The gods must be praised that I am finally saved.
Player: mission
Rorina: Oh, a new explorer? Well how intriguing. Would you like to embark on your first Quest?
Player: yes
Rorina: Great. It just so happens that we had a burglar not long ago that stole one of my finest crystals, and I need you to try and find it and bring it back to me. Sometimes at night I've heard what sounds like footsteps coming from beneath the castle, which is odd because as far as I know there's no basement, but be my guest to look around. Return to me if you manage to find my missing crystal and I will be sure to reward you for your efforts.
Go outside the castle and find the secret basement entrance hidden behind a bush here:
Follow this path to get to the crystal room:
Note: There's another reward at the other mark on the map, through a secret passage.
Report back to Rorina.
Player: hi
Rorina: The gods must be praised that I am finally saved.
Player: glimmering crystal
Rorina: Oh, have you found my precious?
Player: yes
Rorina: You found my missing crystal! How amazing! Thank you very much, and I hope it wasn't too much trouble. Please, accept this as a payment to aid you on your future adventures.