Step one is to totally document your idea on paper. Next, put everything in an envelope and address it to YOURSELF at your home address. Drop it in the mail. When it comes back to you DO NOT OPEN IT. Just put it away. If there ever comes a time when you need to prove that you thought of it first, this postmarked document will come into play.
Next, do a patent search. There are several on-line repositories of patents issued in the US. Do a web search to locate these. It will take a LONG time to conduct the search. My advice is to hire a professional if you have any faith at all in your idea.