best way to get rid of mold in shower?

Depends where it is. On tile and grout, a mildew spray or bleach diluted roughly 1 part to 10 parts water works. Spray it, leave it ten minutes, scrub with a stiff brush, rinse. Window open, and never mix bleach with vinegar or anything with ammonia in it. If the black is inside the silicone along the tub or tray, no cleaner will really shift it, the mould grows into the caulk. Cut the old bead out with a blade, clean and dry the joint, and run a fresh line of mildew-resistant silicone. Keeping it away is mostly airflow. Fan on during the shower and for 20 minutes after, and a quick squeegee on the walls.