Creating a product (invention)

I have (what I think is) a good idea for a product, but I have no idea how to go about producing it. I've made a working prototype out of parts from other items, but I'd really like to reach out to a company or a manufacturing plant and see what I can do in regards to production. At the very least, figuring out how to source raw materials so I don't have to take apart other things to make it.

Does anyone know how to go about this? Where to start, who to talk to, etc? Preferably at the same time, keep my idea safe from someone else stealing it?