Is it the fronts or rears? Only under braking? All types (hard, gentle)? Any squeal at low speed when the wheels are off-centre? From cold or when they are hot? Do the discs clean up after rain?
A mate of mine suffered from squeeling brakes from the rears when hot, off-the-throttle and with a slight turn in. Turns out the 'garage' had used 'high performance' aftermarket parts. A set of Honda pads and discs and the problem has gone.
As others have said dont use 'copper grease' it dries with heat. Take the calipers off and check the pad surface, you make have been sold cheap pads which have glazed. Clean the calipers up and apply specialist brake grease to the sliders, especially on rears. Ask an adult to help.
High performance brake setups should not squeal (unless ceramic and stone cold), if they do they have been poorly installed. A work mate has a F355 with a massive AP Racing setup for track days, even after a hard days work, silent.