1. Let the book rest
When you are just done writing a book, you are attached to it and too personally linked with it. If you are not satisfied with what you have written then you might end up over-criticising it and may never even publish that book and if you are really happy with how your book turned out to be you may end up ignoring the mistakes you have made and even when you have identified the mistake you may not know the solution to those mistakes. So when you let the manuscript rest for some weeks or months when you sit to edit the book you are reading the book from a neutral point of view. You would not feel so attached to the novel and will be able to see it with a fresh pair of eyes in an objective way.
2. Start writing something new
When we finish writing one book, chances are while writing that book more ideas have come to our mind and you can give those thoughts some attention. If you have written a novel it is not necessary that the other book needs to be novel it can be anything: a blog, short story or a poem. You can start researching on some new ideas or just write anything in your journal. Chances are that while writing the first book you learned new things, learned from your mistakes and now is the time to put your learning into practice.
3. Start researching the publishing industry
After you are done writing and editing the book, and you are satisfied with the results. You can publish your book. There is a traditional way of publishing where you need to send your books to the publishing houses or agents. The other way is self-publishing. Many websites now offer self-publishing also. It is necessary you decide whether you want to choose self-publishing or traditional publishing. If you are choosing the traditional publishing way. You should ensure you explore all the options and only then choose a publisher for your book.
4. Read other books like yours
It is necessary you be updated with the literature being published in your genre. So that the story lines do not clash and maybe you can take some inspiration from their books.