Click the Settings (gear shaped) icon in the top right corner of the game screen to change the key settings under [Input].
Skill and item usage defaults to Normal Cast.
If you want to use a skill or item right away without clicking the mouse, change it to [Quick Cast All].
If you don't want to cast them all instantly, you can select them individually and change only those keys to quick cast.
Also, note that you can change the keys that are set by default in [Skill, Interface, Actions, HUD, Communication] by pressing the key that is more convenient for you.