The rear caliper pistons normally require rotating as you push them back,you can buy a special tool to do this, unless you have a a car with an electronic handbrake, then you will need the correct diagnostic equipment to retract the piston and to then align the pads!!!
As you have'nt given the make/model/year it is difficult to give a definative answer
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.