A patent is actually a two-step process. You should start by filing a provisional patent application, and then follow with a regular patent application within a year.
A provisional patent application requires you to present a detailed description of the invention (what it is, how to make it, how to use it, etc.), and high-quality drawings with exact measurements and specific details, along with a $150 application fee.
A regular patent application is more complex and usually requires the help of professional drafters to produce drawings and diagrams, an absolutely clear-cut written description of the invention, and a claim, which specifies how the invention is unique. Wikipedia defines a claim as a description which includes “details that particularly point out and define the scope of the subject matter for which exclusive rights are sought by the patent applicant. The exclusive rights are limited to the subject matter encompassed by the patent's claims.” Claims can be made in the form of a simple, but long sentence, e.g. "A chemical for making hair grow, consisting of the herb chamomile, a 20 percent vitamin E, ..."
For more detailed information, check the U.S. Patent and Trademark Office (USPTO) website, which contains all necessary forms for an application.