the most common cause is build up on the rims from the brake pads and other dirt. the easy solution is to clean them up with some fine, non-soapy steel wool. then wipe with alcohol. (not the drinking kind) if better, but not quite there, try replacing the pads as well.
number two would be toe-in, or lack there of. the brake pads should not be exactly parallel to the rims. the part towards the front of the bike should be perceptibly closer to the rims than the rear, making contact first. if they are parallel, they can shimmy and squeel. if you're not sure about adjusting toe-in, have your lbs or knowlegable friend do it.