Firstly, you need to write something that a publisher would want to publish (it only takes one, but it might take a while to find them. If you are turned down by every single publisher in existence, you will have to consider the possibility that what you have written is not publishable). Next, you need to approach the publisher, either directly, or (which is advisable if you can manage it) by securing an agent who will act on your behalf. The best way to find agents’ and publishers’ addresses is to consult ‘The Writer’s and Artist’s Yearbook’, which is updated every year (Double-check that you are writing to the right person/people; don’t, for example, send science fiction to a publisher of medical textbooks). Wait. Pray. This is the way Harry Potter got published.