When you go to cast a spell that has an AOE ability, you target your opponent and hit your cast key. Then you will see and 'area of effect' button pop up. You can either click that to turn it on/off (on will be glowing sort of, off will not), type /aoe like Cid said or press Z to toggle it on/off on the fly.
-Ceadol Sacamar, Conjurer Extrordinaire!