Hi, thank you for contacting JustAnswer.com. My name is***** will do my best to provide the right answer to your question.
First, when your Kindle is new, it ought to prompt you for basic setup choices such as Language, etc. (If it does not, reset it first, by holding the Power switch/button for 20+ seconds, and waiting for 2 or 3 minutes for it to finally prompt you for basic setup choices, as I said.)
Second, when that's done, you may tap on the antenna icon, to connect to the WiFi. This is necessary before further steps may be taken.
Third, once you're connected to WiFi and to internet through it, you may Register your Kindle device, by going to Settings, Registration (more or less, depending upon the exact model of Kindle used) Register
and hopefully you already have an Amazon.com account to register it to, and if so, choose the option to use an already existing Amazon.com account.
Enter the Amazon.com account email address/username, then the password ***** that account, when prompted for. That ought to get your Kindle registered.
Then you ought to be OK to buy books, or download books already bought from your My Library.
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