You might want to worry about writing your manuscript first and researching your publishing options once you have a first draft of that done. Even if you're thinking of trying to convert a fanfic into an original novel, it's going to take a lot of rewriting, revising, and editing before it's ready to send anywhere.
You should at least know what kind of a story you're writing before checking out different publishing companies, because they tend to specialize in publishing different types of books. Are you wanting to write an adult novel, a young adult novel, or a children's book? Do you know what genre you want to write in? What size novel are you anticipating writing?
You really need to focus on the story itself first, and that means focusing on your writing craft as well. You need to produce a quality, well-written manuscript before you have any hope of getting it published.
