Note:
For pushing in the pistons. NOT ALL pistons are pushed in using a c-clamp. While that's generally the case for front disc brakes, rear disc brakes sometimes need an extra step to retract the pistons. If a c-clamp doesn't easily push in the piston, look on the back of the caliper for a plug. (a bolt that doesn't seem to serve any purpose of holding things together.) Removing that bolt should expose a hole. You'll need an allen wrench to turn a set screw (or whatever it's called) to back off the piston. This is related to the emergency brake function on some rear wheel disc brakes.
Also, if you have brake drums on the rear, it's a little more involved than disc brakes, but still not something that you can't do yourself. There are a couple of springs that can be a pita if you don't have the right tools, but those tools are cheap.
If in doubt, get a (Chilton?) manual that shows you pictures, step by step, how to do it. Available in your auto parts store.