Book publishing is the long game. Thinking of publishing in a short-term way will likely either get you discouraged or frustrated.
Of course, publishing starts with an idea and the desire to communicate it to your community. Once you are ready to act on it, a publisher (likely) needs to come into the picture. Authors may know the names of publishers in their field, usually from going to conference or speaking with their salespeople. But how do you approach them with your idea? I would suggest you start well before any proposal or actual discussion. Developing connections or relationships with publishers can pay off in many ways.