This sounds like something you should let an experienced electrician handle.
Unless you live in a country other than the U.S.... your common hot to ground voltage in your garage is 120 volts. 240v is what you get from phase to phase (hot to hot).
Reguardless, what you'll need to get to 12v or 14.4v is a step down transformer. You'd be able to find it from somewhere like Grainger or MAYBE a radio shack.