You might already be a solid writer, but even the best writers get outside help to hone their skills. A traditional way to train as a writer is to earn a bachelor's degree, and sometimes even a master's degree in English, literature, creative writing, journalism or a related communication field. Budding writers also attend writing workshops and collaborate with other writers through writing groups to get feedback on their work.
Good writers have had lots of practice, both in and out of school. Throughout your career as a writer, write often, whether it's in a journal, on a blog or elsewhere, to help hone your skills. It also helps to put your finished work away for a while and then to return to it later, so you can look at it with a more objective set of eyes. Also, read the work of other quality writers, especially those in your own genre. As you continue to write, you may eventually come up with a solid idea that you can begin to develop into a novel or manuscript.
When you think you've come up with an idea that will sell, the traditional next step is to find an agent. In the big wide world of publishing, an agent will be the representative to guide you and be your book's champion. Start by researching the agents who work with the genre you're working in, and then sending "query" letters to introduce yourself. Your letter should grab the attention of the agent, first and foremost. Then tell some interesting details about the book and share a bit about yourself. In general, unpublished writers should have a full manuscript ready so agents can see what you can do. If you're lucky, an agent will pick up your story right away. If you're less fortunate, it could take dozens of query letters to get any sort of response at all. If you do find an interested agent, she'll want to see your book in its "finished" form, and then will likely send it back for several edits before shopping it around.
Technically speaking, you're not really an "author" until your work is actually published. An agent will help you shop your manuscript around to various publishers in hopes of getting the book sold. Like the query process, this could be hit-or-miss. Your agent could help you publish the book very quickly, it could take numerous tries, or it might not sell at all. If you're familiar with graphic design and marketing, the other option is to self-publish the book as an e-book, or to find and pay a printer to print your book. With that route, you're far less likely to make money or get a large distribution, but you'll be in total control of the process.