Well that can be a number of things-- could be just plain dirty- in which cleaning your brakes will alleviate the squealing. The other and more likely issue is the the brake pads are low and need to be replaced, and if they're low enough to grind, you may need rotors as well. Do you have pictures of the squealing brakes? If not, you should take some, so we can be more specific with the solution.
If they are low, there is a metal tab that will come into contact with the rotor in order to make that "low brakes" squeal. It's saying "fix me."