Allow the shower and knobs to dry completely. Do not attempt to step into a wet shower to prevent a possible fall.
Create a mixture of 1/2 cup white vinegar, 1 tablespoon lemon juice, 1/4 cup baking soda and 2 tablespoons table salt.
Stir the ingredients until they are well incorporated. Dip an unwanted toothbrush into the mixture and scrub the moldy shower knob, paying attention to clean the sides of the knob and the space between it and shower wall.
Allow the mixture to remain for 10 to 15 minutes before rinsing it away with water. Dry the shower knob with a soft cloth to prevent future mold growth.