First you will need to clean the furniture to remove any mold, or mildew that may have been caused by moisture. You can find cleaners at most home and garden stores, such as Home Depot. There are also DIY cleaning solutions that you can make at home with common household chemicals, like vinegar. Then you would need to proceed with a water resistant stain, since this is furniture for outside. You can also find these items at your local home and garden store. Thompsons Water Seal is one brand to try.