#1 - REGISTER your Nook with B&N, BEFORE you remove the old software.
Well, there's this "Dummies"
guide for EMMC that will COMPLETELY remove the Stock B&N software, then there's the "Hybrid/SD Card" method
, that will keep the B&N stock and add a dual boot option with CM 10.1.
CyanogenMod 10.1 is based on Gogle's Jelly Bean OS for mobile devices, modified to run on a number of devices, including our Nook HD/HD+.
I'm using the Nightly CM 10.1, and it runs better than the stock software, there are other ROM available, but I've never tried them so I can't offer any opinion on them.
Following the Dummies guide by e.mote, is simple and straight forward, read the instructions, then read them again, download the required tools and software and you'll be running CM10.1 in a few minutes.
Good Luck, and enjoy your improved tablet!