first thign would be take off the wheel make sure the brake pads arnt too worn.
then you will have to take off the brakes and check or replace the antirattle shims, these stop the brakes from rattling and thus screaching.
then when reinstalling clean and grease the proper components and go for a ride again, problem solved?
also before leaving lightly rough up the rotors in a swirl motion with some sandpaper.