It would be best to get a book on your car. Unfortunately, the usual (Haynes) have one for earlier models that does not really cover the 2003 - even though it says it does.
Block the wheels and make sure the parking brake is OFF.
Remove a bolt holding the parking brake cable and remove the lower bolt on the inside of the wheel where the brake caliper is attached. Then you should be able to rotate the caliper upwards where the pads can be replaced.
At auto parts stores you can get a tool that allows you to rotate the piston back into the brake cylinder, although the manuals say needle nose pliers will work. They might, but not if the brakes haven't been replaced in a long time.
The piston has to be lined up so it is perpendicular to the caliper so the brake pads will fit into the piston grooves at the top.
At least on the front brakes all one has to do is compress the piston.