You can easily assign an attack or spell to your right mouse button. Open your Skill Page by pressing the associated icon in the bottom interface or simply hitting the K key. Left click on the spell you want to set and it will be mapped to your right mouse button. You can also assign a hot key to any of your "Active" spells by placing the mouse cursor over the spell and pressing F1 – F12. This will bind that spell to that key, allowing you to call it up instantly as your right mouse button skill.