When you turned the Kindle on, you had to register it, following the instructions that showed on the screen. If it was locked then your info was already on file. If it was unlocked then you would have to go add the Kindle to a specific Amazon account. On Amazon there is a Kindle help page. Everything you need to know is right there. You have a free trail of Prime and Kindle Unlimited with every new Kindle Fire. Go to
http://www.pixelofink.com/
When you see a book you like click on
Still Free and it will take you right to Amazon to buy it for $0.00. It will automatically be sent to your Kindle.