If you are looking to remove a nail varnish stain then try the following.
You must not try this method on acetate based fabric as this method will damage the fibers. Try and remove as much of the nail varnish / polish as possible. Then you need a non greasy nail varnish remover. Dab the stain from the back of the fabric with the remover, this will help not to spread the stain. Then launder as usual.
If you have a nail varnish / polish stain on your carpet or upholstery then the method is the same except you cannot get to the back of the fabric. Once treated then wash with a carpet or upholstery shampoo.
If the garment is dry clean only then the only solution is to have it dry cleaned.
Here is a tip that was sent into the site that is well worth a try. Put shaving cream on the area soiled with nail polish and gently rub with a cloth, keep repeating until no sign of the stain exists then rinse with water and soak up with a clean cloth. I find an old towel works best.