The time to develop your marketing plan is before you start writing your book. Authors who submit manuscripts to literary agents have to have a marketing plan as part of their proposal. Create a realistic marketing plan (one that's within your budget and your abilities) and take action on it as soon as you have a first draft.
With more than 400,000 new titles--at least half of them self-published--released yearly, the competition is too fierce for you to risk having a book that's in any way less than professional. That means assembling a production team. At a minimum, you'll need a copyeditor, a book designer, a cover designer and a proofreader. For nonfiction books, you might also want to hire an indexer.
Most of the print-on-demand houses offer inexpensive (starting at $500) packages that bundle some of these services together, though what you get is usually a basic template design, not customized service.
You have a choice when it comes to how you want to print your book. Traditional self-publishing means finding a book printer to make an offset print run of a few hundred books (minimum), for which you then have to arrange warehousing and shipping. The advantage is a lower cost per book, which means you can set a lower cover price and still make a profit. You need to order your own ISBN (International Standard Book Number, required for selling a book on Amazon or in any store) directly from R.R. Bowker.
Print-on-demand (POD) technology uses digital printing to make one book at a time, as the books are ordered. Warehousing is not required, but the cost per book is higher, which means either a higher cover price or a lower profit per book. POD houses provide you with an ISBN for a fee. Of the POD houses, Lulu has the lowest upfront costs; iUniverse has one of the best reputations; and BookSurge is owned (and preferred) by Amazon.com.
Distribution can be a challenge for self-published authors of either kind. The large chain bookstores often don't want to deal with self-published authors. If you can get Ingram or Baker & Taylor to accept you as a vendor, then you have a chance at getting into Borders and Barnes & Noble.
If not, independent book distributors can get your book into other outlets. And there's always Amazon, your own website, your local independent bookstore and "back of the room" sales.
Wherever you plan to sell your book, you need to know how you are going to get the books to the buyers. Set up your merchant account so you can take credit cards--or at least get a PayPal account. If you need a fulfillment house to ship the books, be sure to find one before the orders start pouring in.