2nd Job Advancement Test
In order to get your second job, you need to pass the advancement test by collecting 30 Dark Marbles from monsters. Once you reach Level 30, allocate all ability and skill points, talk to the same NPC (job master) for your 1st job and get the letter for the advancement test.
You then need to find the job instructor and he will teleport you to a specical place where there are 2 types of monsters. After collecting 30 Marbles, you need to back to the job instructor again for the "Proof of Hero". Talk to your job master and make 2nd job choice.
Depending on your job class, below is the location of 2nd job instructor and the types of monsters you will face.
Magician
Location: Top of Forest North of Ellinia
Monsters: Curse Eyes and Horned Mushrooms
Warrior
Location: West Rocky Mountain IV (West Of Perion)
Monsters: Fire Boars and Lupins
Bowman
Location: Road to the Dungeon (North of Henesys)
Monsters: Evil eyes, and Zombie Mushrooms
Thief
Location: Construction Site North of Kerning City
Monsters: Blue Mushrooms and Cold Eyes
As there is no potion shop inside, you need to stock up your HP and MP potions or foods.