Start with bleach and water mixed to a 1:3 ratio. If you can't find bleach, use any bathroom cleaning solution which contains bleach. This will not only remove any mould or mildew, but also notably fade the stain itself.
After you seal the leak that caused the water stain in the first place, rinse that area with a solution of water and bleach of 3:1 ratio. This will prevent the growth of mould and mildew. After that, you have to dry the area thoroughly. If the bleach and water solution fails to eradicate water stains, you can repaint.