Cindy...Try BarKeepers Friend...can get at Walmart or most grocery stores.
Depends What They are made off. You can use oven cleaner - if copper or aluminium don't leave on longer than 15 min.
Hi Cindy - This is a great tutorial on how to clean, season, and care for cast iron skillets. Hope it helps! Hugs, Holly
PS - We have a FREE eBook with recipes for all of my homemade and all-natural cleaners. You can download it here --> https://pinkfortitude.com/thank#GreenClean