bleach and elbow grease. If it's under the caulking, you'll have to remove the caulking, clean it really well with bleach, dry it out, recaulk it and make sure that it's completely sealed and dried for 24 hours before using the shower again. Then keep on top of it by cleaning it with a bleach or vinegar based solution.

Yeah, Bleach should do the trick. It's not easy, but it can be done.

What kind of shower do you have? Vinyl, Ceramic, Porcelain?

you can explain more about it because i have no any knowledge about it.