Much of what I discuss here will be old material to people who have had a Kindle for a couple of years, but will be very much appreciated by a new owner. Still, this will provide long-term owners an opportunity to share things they have learned. And, I may share something here others had not discovered.
Each Kindle model has slightly different features and a slightly different control mechanism for entering commands. Check to see which features your choice has before purchasing it. Amazon has comparison charts on its models. Go to the web pages for the other makes (Nook, Sony, etc) to compare their features.
Step 1: The Learning Curve
Amazon has some good helps in its Kindle forums. Very knowledgeable people are good about giving good advice, both online consultants and other users.