A little elaboration. Your Nook books are in epub format. The Paperwhite does not read epub. There are options such as koreader or duokan (not recommended) which can be installed on your Paperwhite and will read epubs. Both involve "jailbreaking" your device, which may not even be a feasible option depending on your firmware version.
The easiest way is to convert your epub books to mobi or, preferably, azw3, the proprietary formats Amazon uses. This is easily accompished by installing ebook management software called Calibre.
http://calibre-ebook.com/
The one remaining complication is that Calibre will not out of the box convert any ebooks infested with DRM (Digital Rights Management), a problem from which virtually all of your Nook books will suffer. To solve this you should visit Apprentice Alf's blog.
https://apprenticealf.wordpress.com/ Once the relevant Calibre tools are installed and configured all you need to do is add your books to Calibre, convert them and transfer what you want to read to your Paperwhite.
Good luck, and if you need assistance don't hesitate to post in this thread. However, be aware that Mobilebook rules allow us to link to Apprentice Alf but assistance beyond this with DRM removal is not permitted on the forums.