1. Why do want to patent your idea?
A patent give the inventor a "right to exclude" others from infringing on his/her ownership rights. Thus, a patent is worth as much as you feel you can spend protecting it. While a patent may discourage others from possible infringement, it still will be up to you to effectively protect your rights, and this could mean a lawsuit.
2. Is it novel and did you invent it?
You cannot receive a patent on an invention which is not new. You must be able to show that your invention was not invented or used by others.
3. Is it obvious?
Prosecuting a patent (having the USPTO issue one) involves not only proving that the invention is novel; but that it is not obvious. As an example, if a patent is held for a step ladder with 5 rungs, you won't be able to get a patent on a step ladder with 6 rungs (even if that was never mentioned in the original patent). Obviousness can also be used by combining two or more inventions. For example: Patent 1 is for a type of key used for computer input; Patent 2 is the use of that key in a computer notebook keyboard; it would be obvious to use that key in a desktop keyboard given the existing patents. Thus, that invention would be deemed obvious.
4. Is it patentable subject matter?
In order for an invention be patentable it must meet the requirements of being an invention which the USPTO deems legally patentable. For example, you can patent a lightbulb - but you can't patent the speed of light. You can patent a tea kettle - but you can't patent the boiling point of water. The most common patents are for "things" or for "manufacturing processes." There are other areas, which may fit your invention and you might want to check more in-depth with the USPTO web-site.
5. Have you already disclosed the invention?
Once you have "disclosed" your invention you have ONE year in which to file a patent, or your rights are relinquished. Determining whether you have disclosed your invention can be tricky. For example, if you try to sell your invention in the U.S., a disclosure was made. If you give a talk to a company, without restriction, a disclosure was made. But, if you give a talk to 500 people and it is done with the knowledge that the information is to be kept confidential, then NO disclosure was made. When in doubt, don't disclose.
Additional Resources
U.S. Patent and Trademark Office (www.uspto.gov)
Note: No attorney-client privilege is created by reading this document. The document is for general knowledge and may not apply to your situation. There is no warranty (express of implied) for the information presented here. Patenting is a very specialized process and you are encouraged to speak with a professional skilled in this area.