Determine what your projected market is and decide the best way to reach that market. If you're writing fiction and want to get a book published as an ebook, there's a good chance your fiction niche has already been captured by a publisher who draws in a large number of readers of that genre. You may want to go with a traditional publisher that publishes straight to electronic book form. Consider submitting your manuscript to print publishers that also buy the rights to publish your book in ebook format. Submission guidelines for many of these publishers are listed in "Writer's Market" or can be found at the publishers' websites.
For a nonfiction ebook, chances are your market will be searching for the subject you are writing about rather than going to an ebook publisher's website and browsing their collections. You may find it advisable to publish your ebook yourself in that case. An additional advantage to self-publishing is that you make money on all the profits from your ebook, not just on book royalties.
For ease of publishing, write the ebook as a Word or Open Office document. Add pictures, page numbers, a table of contents, a title page and copyright page, and an index, if applicable. Once you've made the book professional looking and error-free--and you may wish to hire an editor to polish it for you--convert the document to a .pdf format using special .pdf conversion software, if your word processing program doesn't already have that option. Customers who purchase your book then don't need an ebook reader to read it. Rather, they can view it using free Adobe software or in their Web browser.
Put your ebook up for sale on your own website or blog. Write sales copy and add a shopping cart that allows easy downloads of your ebook. When you get a book published as an ebook, make it competitive by including a thorough description of the contents, allowing your readers to see an excerpt, and including the page count. Market the ebook in relevant forums or just by spreading the word to your colleagues, being sure never to spam anyone.
If you'd rather get some help with the development of your ebook, explore self-publishing options that allow you to get a book published as an ebook in a streamlined process that is relatively inexpensive. Companies such as Lulu.com and BookSurge and eJunkie help you format an ebook and set up the ebook for sale online and even offer print options. There are a variety of print-on-demand services that can allow you to complement your ebook sales.