Depending on the size and type you have, you may be able to buy a new one for $15-20, and then attempt to teach yourself. I've seen them on many online stores pretty cheap. May also help if you post a picture of what you are working with.
Typically yes... they seize up due to galvanic reaction of the aluminum and steel components...
Regular servicing and greasing helps prevent oxidization....
You can likely rebuild it, but the replacement cost is low enough that a rebuild can be more expensive (with pads). I was amazed how much better our brakes worked with two new calipers.