Start with a patent search. If you want to approach a company, you'd best have your IP locked up tight, or they
will steal it. Guaranteed. Step one though, is to make sure that nobody else has come up with the same idea. You can do an initial search of the patent literature using
Google Patents, but all that will tell you is if someone has not already gotten your exact idea. The next step is to contact a patent lawyer, and to get their office to do a proper patent/prior art search, and will be able to give you guidance as to whether or not the idea is patentable. From there, if your idea is patentable and you have a company in mind to contact, then file a Record of Invention, and then once that's approved (i.e. you can officially say that your idea is 'patent pending'), then,
and only then, should you approach the company in question.
It's terrible, but that's the state of things at the moment. If your IP isn't locked down solid, it will get stolen if you go to a company. Depending on your jurisdiction and the scope of your claims, it can cost anywhere from 5-25k (and more if you're close to someone else's IP, or are aiming for very broad claims) to get a full patent. With an ROI you should be somewhat covered as far as getting priority for getting a patent, but you won't be able to license/sell your idea until you get a full patent as you won't properly own the IP. Now, if you're trying to use this as a means of getting a job with said company, that might be different...
