I think I am going to switch from fighter to shadow mage. I have all the gear and gold I need (I am only lvl 15 so I dont need that much lol XD.) I am just wondering what a good build is for a shadow mage, and is this a smart move to switch?

If you're only level 15, why not just create a new mage from scratch and learn the build from the ground up?

The build for most Mages are the same, really... Mental Prowess and Bookworm are a given, Couch Potato is handy if you can afford the slot, if you aren't using BSoL then Gym Freak and maybe Average BMI... Ice Bolt is almost always there, perhaps an elemental boosting spell for your element, and support spells like Spirit of the Hare, ShadowWord: Smackdown, Righteous Stomp, Light in the Loafers, the Chill Bill spells...

Main problem would be that, being a Pure, you might not have many skills available to you. But that might not be a problem. Personally, the few Mage Pures I've met had been END Pyromancers. I guess the range of Fire Bolt plus their unholy Endurance is their main advantage.