There are many age-old and homemade methods to clean the mildew from various fabrics. Lemon juice and salt is probably the most common and oldest method of cleaning the mildew from fabric. As soon as you discover there is mildew on your clothes, first step would be to brush off the surface immediately. Mildew is actually a living organism, a mold that thrives in places that are warm and lives longer in humid environment.
Take the piece of clothe/s away from rest of the clothing and take it to area where you can brush off the mold growth from the cloth. You can use a soft bristle brush to do this or you can just do it with your gloved hand. If you delay in brushing off the mold, there are high chances that the mold will eat your clothes away and soon you will find small holes in the place of the mold. Take enough care not to scatter the mildew spores in your house or anywhere on the clothes you are wearing. Sometimes when the mildew hasn't grown much just cleaning it with a brush and washing it in a detergent and drying in the sun would be sufficient. But even after dusting the mold from your cloth if you still find the mildew spores on the cloth then you can use any of the following remedies listed here.
Mixtureof lemon juice and salt as mentioned before is the safest and easiest method to clean the mold from your clothes. Make a thin paste of lemon juice and salt in a bowl and slowly spread it on the area where you see the mildew spots. Allow the clothes to dry in the sun and once it is dry, rinse it under the running tap water and sun-dry it again. The mixture of lemon juice and salt works as a natural bleach to remove the mildew and the mold stains from the fabric.
If the mildew stains are stubborn and are spread wider on the fabric then try using Peroxygen bleach to clean it. Take a pint of water and make a mixture using about one to two tablespoons of sodium perborate or powdered bleach that has sodium perborate also in it. If you do not have powdered bleach with sodium perborate then alternatively you can use same amount of potassium monopersulfate. If your fabric can be washed with hot water, use hot water to soak the cloth with the mildew stains and apply this mixture of powdered chemicals. You can also soak the cloth directly into the solution for at least 30 minutes and then rinse it off thoroughly with cold water and allow it to dry in sun. If the stains are old and are not gone after the first wash then you might have to soak the fabric in the solution for one night and then follow the same procedure. Only risk you have with this method of cleaning is that you might end up harming the fabric texture and color. Hence ensure that you check your clothes for colorfastness before trying this method.