All classes in Flyff Legacy are damage dealers, so you will have to use potions to restore your health in combat. The game offers an auto-potion system which will consume potions automatically based on your preferences.
Tap the ↗ menu and then select the "Auto" button. You can then configure when and how the auto potions will be consumed.
In 'Auto-Potion Usage', you can select at what moment the potion will used, note that the % is proportional to your character's current life. Note that the potions have a cooldown period, and won't be used when cooling down.
In 'Combat Settings' you can also choose to consume the smallest potions first.