I would recommend that you do not build a prototype or try to market your idea yet. You should speak to a patent attorney and have the invention patented first. This will give your idea some legal protection from being stolen from you.
I would recommend NOT discussing the idea in web forums or even in person with anyone until you have talked to a patent attorney.
Do not waste your time and money on so-called patent mills that promise to patent your idea for a few hundred dollars, make sure to talk to a real live attorney who can advise you on the cost and process of obtaining a patent.
It is not a cheap process. Here is a link to help you locate a patent attorney if you so desire:
http://smallbusiness.findlaw.com/patent/ Best of Luck$$