How can I be sure someone won’t “steal” my idea if I submit it to your service?

Current copyright law protects your work the moment it’s created. Go to http://www.copyright.gov/help/faq/ for more information. This site will also tell you how you can officially register your manuscript under copyright laws. Keep in mind that very few ideas are totally unique. What may seem like a unique idea is actually more common than the victim of theft thinks. (There are plot books with supposedly all the available plots concepts for novels!) In other words, what someone thinks is theft of their idea is actually not. If you are still concerned, consult a copyright attorney.