Skills are categorized into Active Skills and Passive skills, and Active skills can be set to automatic.

You can check all the skills and the skills you've learned from the Skills menu at the top right,

and you can set the Active Skills you've learned to 3 automatic settings by tapping the icon on the right of the skill name. 

 - Auto: Automatically use the skill for all attacks.

 - PvE: Automatically use the skill for PvE attacks only.

 - PvP: Automatically use the skill for PvP attacks only.