Your first step would be to get advice from a patent attorney. There are some scams online concerning patents where you pay for the patent and you somehow get ripped off.
If your item is patentable, you would perform a patent search to be sure it doesn't already exist. Then you would apply to the U.S. Patent and Trademark Office.
Document details (description, pictures, etc.) of how your idea works and how it is unique and contact your closest USPTO (United States Patent and Trademark Office)
The first step is to check to see if the idea has already been patented. You can check existing patents as well as find out the complete process by visiting the Patent office website.