Using skills:

Basic attacks: Every character is capable of using basic attacks at no cost.

Skills: Every character has skills and most of them cost energy to use, but some are free of charge.

Passive skills: These doesn’t have to be manually used, but are triggered passively when certain conditions are met.

The default battle setting [Skill First], which means you use a skill and then select a target. You can change this into [Target First], then you need to target an enemy before tapping a skill. You can change between these two modes at will by tapping your Avatar in the Lobby and selecting [Target First] or [Skill First] in the Settings.