Review and edit your work until you are completely satisfied and love it. Make sure that your work is the best that it can be. Whether you've written a novel or a short science fiction story, review it and re-write it until you are absolutely certain it is perfect. And then leave it alone for a week and review it again. It's amazing how many times you might change your work until you know it's ready.
Ask your friends to read your writing. Get a feel as to how they interpret your work. They may find mistakes you've missed or give you an idea of what your work might be lacking. Their honesty can only make your work stronger.
Find a literary agent to represent you. A literary agent is someone who has more pull with the publishing houses. You do not pay them directly; however, they take a cut of the money you'd make on your book. Their job is to try to sell your manuscript to a publishing house, handle contract negotiations and stand as a buffer between you and the publisher. An agent will shop your manuscript around publishing houses, using their inside knowledge to place it with the right editor.
Find out who the agents are for writers in your genre and then search the Internet for their homepages. Submit the first three chapters, a brief (1 or 2 page) synopsis and a short cover letter asking them to represent you. If they feel you have a knack for writing, they just might want to represent you.
Browse the Internet and search ITI's Literary Marketplace (see Resources) for lists of publishers if you decide to deal directly with publishers. Sign up for free with ITI's Literary Marketplace to view thousands of publishers by genre.
Read each publisher's submission guidelines carefully and follow them. Some will require that you send them a "query" letter first via email. This is basically an introduction of yourself and a short synopsis of your work. Others will require a literary agent contact them directly.
Wait for a publisher's response. Have patience and keep writing during this time. You may wait for three to six months to hear from a publisher. If they do not accept your submission then try another publisher. Keep submitting and don't give up.
Publish your own writing if you are not satisfied with the responses of the publishing houses. Or maybe you just don't want to wait.
You can self-publish for free at websites like Lulu.com and Createspace.com. It's very easy and you can get published right away. You will be amazed at how easy it is and what a professional job they do.
Once you publish your own book you will want to market it. Create a website and introduce yourself as well as pump up the hype on your book. Don't forget to list your website and book location on Google and Yahoo's site submission so that people can find you (see Resources).