Squeaking brakes is not a big deal. It happens on older bikes all the time. Take a look down into your brake caliper and make sure your pads are good, if they are, then you may have 1 of 2 problems:
Your brake components have become delaminated, basically they have been rubbed smooth and squeal when they grip. The repair is replacement of the pads or rotors or both.
Second, older brake calipers and pads will not hold onto the pads as tightly as they once did, so the pads will actually vibrate a little and emit a high pitched noise, like a violin vibrating the strings. The repair here is to use some brake quiet on the back of the pad to dampen the vibration.
