Once you complete the level 10 guild quest you will have the ability to change your class but visiting any on the other class guilds in the 3 cities. In order to obtain your job you will need to have leveled your class to 30 completed all the class quest from your guild and leveled the secondary job to level 15. So in your case I'm assuming you are leveling gladiator and want paladin. Just get your gladiator to level 30 and do all the gladiators guild quests. Then go to gridania get conjuerer and level it to 15 and then you can go and unlock paladin.