When I work on disc brakes one of the first things I do to eliminate squeal is make sure the brake is clean. Dawn dish detergent and water work great for this.
.5 tsp per 24oz works great for me. It helps to have a repair stand also.
1. Pull your wheel out and carefully push your pads and pistons in with an 8mm box end wrench. Then spray inside the caliper with the dish washing solution. Flossing through the brake caliper with a scoth brite pad will help brake up any dust collected in the brake and on the face of the pads. Irrigate the caliper with a few more sprays of solution.
2. Next spray the rotor and lightly scrub with the scoth brite pad. You want things to get a little frothy, suds help lift away the contaminents. Then irrigate with a few sprays and replace the wheel.
3. With the wheel installed, spray the whole assembley (brake, rotor, frame and hub) anything in the general area, the solution won't hurt anything. Moving the wheel, either by pedally the rear or spinning the front you want to pump the brake a few times with the wheel moving. All the time keepin an eye on the suds building up in the brake. Keep doing this a while till the gray suds are replaced by white suds. Irrigate with a few squirts of fresh fluid and then let it dry :)
The Dawn solution does not need to be rinsed off and it won't harm any seals or brake components. After a few stops on your initial ride the brake power should devolop pretty quickly. And your squeal should go away if it is due to lite contamination. There could be other reasons for squealing but this is a good start.