Use a clean rag with disc brake cleaner or cheaper isopropyl alcohol, to rid your rotors of oily deposits that can cause squealing and also kill your pads. Don’t use general bike cleaner or detergent because they may leave deposits of their own.
Generally, it’s good not to mess too much with the rotors or pads, but if you’ve accidentally used spray oil inside your workshop, or got drivetrain cleaner on your discs, or you've been transporting your bike on the back of your car where the rotors are near exhaust and road contaminants, giving the rotors a good clean may get rid of the squealing.