Reducing humidity through proper ventilation is an excellent strategy to control mold. If you already have a problem with mold, you need to address mold cleanup as the first step. The U.S. Environmental Protection Agency offers a free brochure you will find helpful. To install a ventilation fan, check the installation instructions. Try to locate the switch for the fan as far away from the tub/shower as possible. Most codes require that the switch not be accessible when standing in the tub/shower. A fan or fan/light can typically be installed on an existing light circuit but it is best to have an electrician confirm this. If it’s a fan/light/heater combo, it will need a dedicated circuit back to the panel.