I apologize for not being more specific, but there are many types of mold that can affect plants. The best method to treat it depends on the type of mold present. Giving your plants the appropriate amount of sunlight it requires, and avoid overwatering are the best ways to present mildew.
I have used a 1:10 mixture of whole milk and water to treat powdery mildew, and it works very well. However, it works best as a preventative; you may need to treat affected plants twice, and remove any badly damaged leaves. Spray the plant to run off, making sure to cover the undersides of the leaves as well.