Search these forums a bit for spell information, and just browse through the spell editor. You could have found out how to make a custom spell on one of the threads on the front page in this forum.
JASS is WC3 code --- it isn't nessacary in the slightest to have it. Learn the Graphical User Interface triggers first
I don't think that using WEU in itself will help you much, but it will be beneficial to have the extra trigger, model, and customization options.
To make a triggered spell:
Events: Unit-A unit casts a spell
Conditions: Spell Being Cast=[your custom, or not, spell] ; Casting Unit=[either a specific unit, or unit type or whatever]
Actions: [put your actions here~common ones are special effects]
Just browse through the tutorials on our tutorial site about triggers first, study the object editor, and then search this forum for spells and triggered spells.
Actually, I believe they do have a JASS editor. I think I saw something about it at wc3campaigns ...