An idea must be tangible to be patentable. One of the implications of this principle is that your "invention" must not be too abstract -- a law of nature or a mathematical theorem, for example, cannot be patented. Your invention must perform a practical function, and you must be able to describe it in enough detail to allow someone else to manufacture it. Patent applications typically contain precise specifications and drawings.
In addition to practical usefulness, your invention must be unique. This means that no one may have ever invented it before, regardless of whether the previous inventor applied for patent protection or not. Your invention must have also required a leap of creativity, known as "non-obviousness." This means that your invention cannot be patented if, from the point of view of a competent specialist in the field to which your invention belongs, its primary innovation would be an obvious next step in the development of the state of the art.
To patent your invention, you must reduce it to specifications and drawings. You must also create formal legal claims that establish its novelty, usefulness and non-obviousness. These claims are highly formalized and should be drafted by a patent lawyer. Your specifications, drawings and claims must be included in the application you file with the U.S. Patent and Trademark Office. The USPTO will review your application and will probably request a number of revisions. It will publish your application 18 months after your application filing date. Although approval may take years, approval is granted retroactive to your filing date, meaning that once you have a patent you can sue for infringement that occurred any time after your filing date. Most patents endure for 20 years after the filing date.
There is no such thing as a global patent -- you must apply for patent protection in every nation where you desire protection. There are international treaties, however, such as the Paris Convention for the Protection of Industrial Property and the Patent Cooperation Treaty, each signed by over 100 nations, that make it easier for you to patent your invention abroad. The Paris Convention guarantees that your application will receive equal treatment with local applications. The Patent Cooperation Treaty allows you to have the preliminary steps in the patent examination process performed by filing a single application.