The first ting you need to do is make sure your idea isn't already on the market. Do a patent search yourself on the http://www.uspto.gov/
website. This may take some time and effort on your part, but it is free and will save you further work on an idea that is already patented.
If you don't find anything in your patent search I would then narrow down which companies you feel would be interested in your idea. This may sound dumb, but a number of Inventors will send an idea to a company just because they like the company, not because the company makes similar products. You don't send Black and Decker ideas for dog collars.
Once you have gotten a list of companies together you need to contact them and ask if they look at outside ideas from Inventors. If they do they should send you a nondisclosure form to fill out prior to sending them any material
While you are waiting for the NDA you need to work on your pitch. You can see an example at viewtopic.php?p=6124#6124
Make your pitch short and concise pointing out your ideas benefits and target market. Once you are satisfied you have the pitch ready you send it to them for review. If they are interested you will receive further documentaion from them about licensing. Make sure you READ the fine print and have a lawyer look it over if necessary to make sure you are understanding the terms of the agreement.
Once the agreement is signed you are going to have to wait about 6 months to a year for your product to hit the store shelves depending on how fast they can get things rolling.