Open an account on Amazon's website (see "Resources" below) or sign into your existing account using your email and password. Before purchasing and reading a book from Amazon, you'll need to download the Kindle app or software for your mobile device.
Go to the Kindle application page (see "Resources" below). After the page finishes loading, you should see an image with the words "Read Anywhere with Our Free Reading Apps" on it. To the right of the image is a list of digital devices for which the company's software is suitable. Click on the device you're using to download the software.
Read over the minimum system requirements to make sure that your device qualifies and then click the orange "Download" button. Follow the on-screen instructions for installation. After the software finishes installing, you can start purchasing and reading Kindle-format ebooks.
Go to Barnes & Noble's website (see "Resources" below) and create an account. Log in using your email address and password.
Click on the "Nookbooks" tab at the top of the screen and then on the "Get Started" button on the following page to reach the Nook applications page. There you'll find a list of devices for which Barnes & Noble offers applications.
Read over the list of devices in the left-hand column to find your device. Click on the name of the device you're using for more details as well as screenshots of the application in use.
Click the large green "Download Nook For (device name)" button in the top right corner to begin your download. After installation has completed, you can begin to browse Barnes & Noble's library.
Click on the "Nookbooks" tab again to be sent to the library's main page. Here you'll find new releases and editor picks. You can browse by selecting a category from the left menu or you can enter a title for which you're looking in the "Search" field at the top of your screen. After you add an item to your cart and have paid for it, you can begin your download.
Create an account on Smashwords (see "Resources" below) and then log in using your email address and password. Smashwords offers books in various formats, including Kindle, Online Reading, PDF and RTF.
Browse the site's selection by choosing a category and perusing the titles or by typing the title of a book that you'd like to purchase in the "Search" bar in the upper right corner.
Click on the title after you've located it to examine a full synopsis, parental rating and price. To purchase the title, scroll down the screen until you see the list of available formats.
Click on the "Buy" link beside the format that you'd like to use on your mobile device. Follow the on-screen instructions to check out. Your download will begin after you've made your purchase.