Subclasses go in the same way as your main class. You choose the other classes that you think bests fit you with your first class. Some players don't know what classes to choose for their main ones because they probably have two or three favorites. Having a subclass is a good chance for a player to experience the different skills and combos.
Players can always change their subclass whenever they like every 10 minutes. To reset, simply go to the skill interface (hotkey K) and press the "Reset" button to change subclass. Note that players can't reset if they are in combat.
While players will use their chosen class’ weapon (and the skills that come with it) for the beginning chapters of their journey, a whole new world of potential is unlocked at level 40. At this point you’ll be able to select a secondary weapon, normally wielded by one of the remaining classes. You can then train up your character with the secondary weapon the same way you did with your primary.
Choosing a subclass cannot be the same class as your main one! For instance, if your main weapon is a Sword and Shield, you have to pick one out of the other weapons. You can't pick sword and shield once again.
A player could learn a subclass skill (primary skill) of the secondary class. You could visit the Skills page for all types of skills. You could also make a combination of your first and second class together and learn new skills. Keep in note that subclass skills have 3 times longer cooldown compared to the subclass as primary class.