This article published by Huffington Post is one of the best I've read on what to do before you self-publish or even submit your manuscript to an agent or editor.
He points out the importance of having your book edited by a professional. Did you know there are several kinds of editors and they sometimes do several kinds of editing, but some do only one kind of editing. Do you want copy editing or content editing or both and more?
David Kudler, author of the article on Huff Post, is an editor and he knows what editors see in the manuscripts sent to them. I have had authors call me on the night they finished writing their last word in their book and say, "Now I'm ready to publish."
Sadly, their work has just begun. Even professional writers need editors. Even editors who have worked on one manuscript for a long time need new eyes to read it again. I see many books today that have so many errors in them I can't finish reading the book. I hear this from other readers who are amazed at the poorly edited books sold in today's market.
Don' t let your book be one of them. Don't publish a book that you will be ashamed of in a year or two when you learn the craft of writing.
Just like I can't decide today that I want to be a brain surgeon because I have watched videos and I am sure I know what to do, a person can't sit down and write a good book if he or she has not had any education in the craft of writing. People who do that often turn out poorly written books and only their family or friends will buy it. Take writing classes. If none are being taught in your area, take online classes. Be sure you research and know your instructor is qualified.
Be aware that you will likely do many revisions before you book is ready to publish. Don't give up. Let experienced editors help you make your book the best it can be.