Should be pretty easy to find a Sun Tour FD on epay, if you need to replace it. Not sure what kind of controls you have, but put it at the lowest gear and make sure the cable is snug, but not too tight. Also make sure that the derailleur moves freely (easier with the cable disconnected). If the cage is centered over the lowest chainring and it still won't shift, something is farked.
Park Tools has some excellent repair help articles online:
Park Tools front derailleur adjustment