A Novice who has reached Job Level 10 can become a Thief.
Before attempting the test, make sure to allocate your skill points.

1. Near the desert city of Morroc, there is a pyramid. On the first floor, in the center, you’ll find the entrance to the Thief Guild /navi moc_prydb1 39/129.

To get there:

Travel to Morroc. Enter the upper-left portal. In the northwest, you’ll find the entrance to the pyramid. Once inside, move diagonally toward the center.

Enter the corridor, move forward, then turn left. You will reach a small room where a Thief Guide will explain that to join, you must pass a test stealing mushrooms from a mushroom farm. Don’t worry about the morality of it; the owner is known throughout Morroc for being spiteful and greedy.

2. Leave the pyramid and find a Mr. Irrelevant nearby /navi moc_ruins 141/125, who will show you the way to the farm. To pass the test successfully, you must earn at least 25 points:

Orange Net Mushroom - 3 points

Orange Gooey Mushroom - 1 point

3. Beware of monsters in the area, some of which are aggressive. It is recommended to bring healing potions.

After collecting the required amount of mushrooms, return to the Thief Guild. This time, speak to Comrade /navi moc_prydb1 42/133.

He will count your points, and if you have passed the test, you will be accepted into the Thief Guild and gain the Thief Job.