To solve this squeaky noise, we advise you to visit a professional bike shop. Otherwise, there's usually three reasons which you can try to fix yourself: 

• Contamination or glazing of the brake pads or rotor can be the cause. Contaminants (such as oil from our fingers when you accidentally touch the pads or rotor) cause the pads to lose grip on the rotor, which can cause vibrations leading to squealing. Disc brakes can be cleaned with specialized cleaners. Never use oils or detergents. 
• The actual set-up of the brake. If the pads aren't aligned properly, it can cause noise-inducing vibrations. The same can happen if bolts and other fitments aren't tightened correctly. There is a simple trick that will adjust the brakes in the right place. The first step is to loosen the Allen bolts a little when you spin the wheel and "brake". Keep the brake pressed with one hand and tighten the bolts, when the bolts are tight you can loosen the grip on the brakes, and it should be in the right position. If this does not work you can move it around a little while spinning the wheel, if you don't hear it anymore it's adjusted right. Don't touch the braking disc or wheel while spinning, you can hurt yourself as well as the braking disc, which cannot handle the human touch and will rust. 
• The brake pads are worn and need to be replaced.