I have an invention. What to do next?

I have an idea tha I know will sell. What do I do next? I've heard of these companys to send your ideas to but I'm also aware that some of these companys just take your ideas and cut you out of the deal. What is a good reliable company I should send my idea to and if there is not one what should I do next? Do I need to make a promo of my Idea? What if I don't know how to make a promo? How should I go about this? Help please!