Krud Kutter. Then let your shower dry out before closing it up. An exhaust fan will.help p revent, or a dehumidifier.
There are many products out there for this purpose and work better than anything you can make...I would look at the home improvement stores...in their plumbing department.
I personally find bleach the best.
White vinegar is my go to. Vinegar kills the mold spores, bleach does not. Check this out.