Write and complete your manuscript before you do anything. Writing a book is a long and difficult process and is only the first step. You will probably want to do rewrites and possibly get it edited by a friend or even a professional editor.
Decide on what kind of format you want to publish in. You could leave your book in digital format or get it printed as a hard cover, a paperback or even something more informal like loose leaf held together by a ringed binder.
Consider hiring a graphic artist or graphic designer to create a cover and any other graphics for you. A cover makes a book seem more professional, and though there is nothing wrong with a cover composed of just text, there are many freelance design websites, like CrowdSpring for instance, that will allow you to name your price and what you want your cover to look like.
Find a publisher for your book. Services like Lulu or Blurb will allow you to publish your work in a number of different formats, and a service like Booksurge will allow you to self-publish and make your book available in the Amazon marketplace, where it has the potential to be considered by thousands of customers.
Tell your friends and family about your book. You can have them purchase copies of your book or purchase them on their behalf and send them to them. Get feedback and start working on the next book.