You can protect your idea by filing a provisional claim with the USPTO. This is kind of like a "patent lite" and allows you to protect the product for up to a year. The cost is US$200 and should be done straight away. Visit the PTO website for information on filing applications electronically.
After that, prepare yourself for a tremendous uphill battle. If this is a design patent (i.e. a new form of an existing product), you will find it difficult to locate interested buyers. If this is a utility patent (a new product or process), you will need to find companies within the industry and see if they are interested.
You will want to get a hold of a standard NDA/NC (Non-Disclosure Agreement / Non-Compete). These are fairly boilerplate and should be easy to locate on the 'net. Requiring NDA's is STANDARD OPERATING PROCEDURE when shopping around an idea and manufacturers are generally comfortable signing them. If a manufacturer refuses to sign, WALK AWAY. There is something wrong. An NDA is no guarantee that a company won't steal your idea, but at least it provides evidence of due dilligence during any potential infringement lawsuit.
Search the various government department websites for grant applications. The government is always looking to foster the development of new ideas that can potentially benefit the nation. There are grants for both non-profit and for-profit company's available and some even include grants for "concept development and R&D".
Barring that, you may want to consider some form of rapid prototyping. Depending on the invention idea, this could be a short production run of the product, some kind of stereo lithography or other RP, or a "proof of concept" 3-D model, engineering analysis, etc. Having a physical, working prototype that a potential buyer can see, touch and interact with is FAR more impressive than a napkin sketch. Be prepared to spend $1-10K on a prototype. Well worth the investment, because if a picture is worth 1,000 words, a prototype is worth 1,000,000.
Good Luck!