Registering to Share Your Kindle Books
Input your user details into the Kindle using the keyboard. If your user details contain non-alphanumeric characters that you don't see on the keyboard, these can be accessed by pressing "Sym" to see a menu of all other characters available for input. If you've forgotten your user details, you can retrieve them by visiting Amazon.com on your computer.
If you find the keyboard cumbersome to navigate, you can also register a new Kindle online. Go to this page if you're anywhere EXCEPT the United Kingdom (including the USA), or this one if you're a UK customer. Input the serial number from the rear of your Kindle (found near the bottom of the device) and click "Register."
(Image credits: Amazon.com, edited by the author - https://images-na.ssl-images-amazon.com/images/G/01/x-locale/cs/help/images/howto/image010.gif and https://www.amazon.com/gp/digital/fiona/manage?ie=UTF8&ref_=ya_14&)