NY state intellectual property law, seeking patent protection on invention idea, USPTO application

One can protect the description of an invention from being copied by using the copyright that automatically is generated when a work of authorship is fixed in a tangible medium of expression.

One may use the law of trade secrets by disclosing the invention or idea only to those who agree to keep the disclosure confidential and not use it themselves. Such an agreement is known as a confidentiality agreement or a nondisclosure agreement (NDA). Yes, an attorney could draw up an NDA for one's specific situation.

But an NDA would not protect against the possibility that someone might later independently come up with the same invention. If the invention were protected by a valid patent, it would be protected against the possibility of someone later re-inventing it.

Patents are designed to protect inventions (the combination of the idea for a solution along with the knowledge of how to reduce the solution to practice).
Other forms of intellectual property (trademarks and copyrights) are not designed to protect ideas. Rather, they are designed to protect the goodwill associated with a source of good or services (and an associated brand) - trademark; or to protect a create work fixed in a tangible medium - copyright.
Trade secret law may go a ways towards protecting an idea that is to remain secret, but once a trade secret is no longer secret, the idea behind it may no longer be protectable.
If cost is an issue, a provisional patent application is a viable option to preserve the rights to the invention until further funds can be acquired to have a full non-provisional patent application drafted (up to one year).
Please see the Avvo guide on provisional patent applications for further details. http://www.avvo.com/legal-guides/ugc/provisional-patent-applications

Adam Philipp's advice is sound, that the only secure legal protection for an invention (or idea relating to an invention) is under patent law.

There are a few problems with the other advice offered, however. The Copyright Act specifically states that it won't cover this situation: "In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work." So registering the copyright in a description of your idea only protects those particular words (at best), but certainly not the underlying idea itself.

Also, a confidentiality or non-disturbance agreement is an agreement that you would enter into with a particular party (e.g., a potential financier for the project, a company negotiating to produce the invention, etc.). Only that specific party is contractually bound not to disclose the idea to anyone else. The agreement doesn't give you confidentiality over anyone else who learns of your invention or, as Gerry Elman points out, independently invents the same thing by him/herself.

THE INFORMATION PRESENTED HERE IS GENERAL IN NATURE AND IS NOT INTENDED, NOR SHOULD IT BE CONSTRUED, AS LEGAL ADVICE. THIS POSTING DOES NOT CREATE ANY ATTORNEY-CLIENT RELATIONSHIP WITH THE AUTHOR (WHO IS ONLY ADMITTED TO PRACTICE IN THE STATES OF CALIFORNIA AND NEW YORK). FOR SPECIFIC ADVICE ABOUT YOUR PARTICULAR SITUATION, CONSULT YOUR ATTORNEY.