I've been asked a lot for writing advice, and I thought I would put it here for anyone else who was interested. Publishing a book has been a great experience for me, but it has been a long road.
1- Writing a book is the first challenge and 99% of people who want to write a book never finish. So, you're almost there by just writing a book. My most recent published book is my 4th book I've actually written (2nd one published). My first 2 need some work, but I like to write so I continue to write.
2- Next is to do rewrites and get some feedback from your book. Have some people read your book and tell you honestly what they think. Family will be kind and nice, so maybe try to find some neutral feedback. You can find beta readers online to read your book. You can also join a local writer's group to get feedback. This is where rewrite is a big part of the writing process. Many authors rewrite a dozen times before submitting.
3- The next challenge is finding an agent/publisher. Of the people who write a book, 99% of them never get their book published. Every author does it differently. I read a lot of books that are the same genre and style of my book and I wrote to those publishers to see if they would be interested in my book. Many publishing companies require you to go through an agent, while some others don't. My publisher didn't require an agent. A great book to buy to see what agents/publishers are out there is: Jeff Herman's Guide to Book Publishers, Editors, and Literary Agents
4- This is about signing a contract and going through edits with your publisher. My publisher assigned an editor to my book and together we have gone through on at least 6 edits of my book (this is not counting the 3 rewrites I did prior to submitting my manuscript).
5- You must follow the guidelines of your publisher. Most publishers have their own illustrators to do the cover. They just want your book. Period. Publishers also have a calendar of when your book will be released and that is subject to change.
6- Last is to promote your book. Facebook, twitter, blog. Start a blog or website now and talk about the writing process. Get people excited about your upcoming book. I hope this information helps. I wish the best of luck to you.