Once plaster in sheet rock gets wet, it is ruined, and there is no way to patch it or save it. For someone who has never worked with drywall, a ceiling is a very difficult place to begin. First the old ceiling has to be removed. Dirty and ugly job at best. Then new drywall sheets have to be installed. Darned hard work over your head. Then it has to be taped and sanded. Messy, especially for a novice. This may very well be a job for a professional drywaller. They can get the job done quickly and with a minimum of mess.