The most common cleaners used are Oxiclean and PBW (Powdered Brewery Wash). Oxiclean is fairly similiar to PBW and will work fine and is much cheaper. You can search for the two cleaners and make your own decision, but many of us use oxiclean and are completely happy with it.
Fill the carboy up with hot tap water, put a scoop of the cleaner in, and let it soak for several hours, then rinse well. If the carboy is heavily soiled it may require a second treatment, but it usually doesn't. That's it.