If its a rotating squeak, then you have dirt/corrosion between rotor and hub. Also check that the alignment shims (thin metal shims on the sliders- If it has any are properly fitted). If it's a squeal on application then remove the caliper/pads and clean the corrosion off the caliper "contact" area (where it touches the back of the brake pads) use metal brush,rasp, whatever you fancy; but do it!. I call them the "fingers"
for lack of better term. This area has to have perfect contact with the pad or squeels on (light) brake application will result. Shims and pastes are also available as Aids - but start with the basics of a brake job.
If its more of a raspy grind/squeal all the time- check that the backing plate is not touching/rubbing the rotor