Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
Are you referring to the screen unlock PIN code ?
If you are setting the PIN code for first time, then you can setup any PIN code. If you forget your screen unlock PIN code, then you can unlock it from Amazon account (using your computer)
This will unlock the Kindle if it is locked. If that didn't work, then you have to factory reset your Kindle to get access back to it. Enter some incorrect PIN for 5+ times consecutively and this will ask you to reset it. So reset it. After resetting, you can register it with your Amazon account to redownload your books and contents back to it.
If you have any difficulty on following the steps above, please let me know and I will continue to help you