To start on the road to being a published writer you should have a manuscript or at least a well conceived and articulated outline of your book. It is recommended to have a completed draft or have several chapters complete so you can send it to potential publishers.
Depending on what you write, whether it is fiction, nonfiction, memoir, prose, poetry, scripts, and so on, it may limit or enhance your possibilities for publishing. It is difficult to get a complete book of poetry published. Many people who write poetry end up self publishing chapbooks.
An excellent resource for writers is the free website Duotrope.com. This site allows you to search for a home for your polished piece of writing or a manuscript. Duotrope lists information about hundreds of publications, including their pay scale, accepted genres of writing, preferences, acceptance to rejection ratio, the average time length for reply, query method, and more. Additionally, it has a tracking system for all stories, poems, and manuscripts that you submit, so that you are able to track all of your writing submissions.
Using Duotrope to search for publications looking for submissions is an excellent way to find out about writing and to get your work published. This site is a writer's resource. Make use of it and get yourself published!