I'll second what livramax says. Read through the CM 10 posts in this forum to get some background in μSD card rooting. Assuming you have access to a windows PC, the software necessary to create a bootable μSD card is free, with the exception of Winrar ($29) but it has a 40 day free trial period. The cyanogenmod 10 image is available at http://iamafanof.wordpress.com/2012/...r-nook-tablet/
Format your 8 GB μSD card with SDFormatter, download the disk image and write it to the μSD card with Winimage, put it in your nook's μSD slot and boot the Nook. One of the apps included with the image is Google Playstore. Go to the Playstore and search for and download the Kindle app. While you're at it, download the Nook app as well so you can install and read .epub format books as well.
The CM 10 image download will only use 2 GB of your 8 GB μSD card but it's usable to familiarize yourself with CM 10 and the Kindle and Nook android apps. Later you can expand the partitions on the μSD card to use its full capacity.