Here is a helpful list of suggestions if you wish to publish your book:
1. Write, obviously!
2. Read multiple books similar the one you are writing. It will give you a good idea of what is out there and help you develop a critical eye for your own work.
3. Have your work read by others not related to you or your best friend.
4. Join a critique group. There are numerous ones out there, just do a Google search for critique groups in your area or online critique groups. Meetup.com has a number of them, as well (not a sponsor, just like it as a resource!).
5. Beta Readers- for your finished book or to read chapter by chapter. People who will read with a critical eye, making sure that you are staying on topic with your story.
6. Professional Editing- This is an absolute must, even if it is just for copy editing to ensure that your punctuation, grammar, spelling, style and structure are error free. A copy editor will charge either by the word ($0.014 average) or by the page ($3-4.50 average), but it will be by all words or pages, not just those they corrected!