Most of the answers on this thread seem to only deal with the design and ebook conversion side, which you should get professional help with. It's the same with a printed book. If you do it yourself it will more than likely have poor layout and seem self-published and unprofessional.
The real question, though, is not how to covert an ebook, but how to distribute it on the Nook. For that you need to register as a publisher through Barnes and Noble. There's information on their website about it here: http://www.barnesandnoble.com/ebooks/help-faqs.asp
