Registering your work for copyright protection is virtually the same if it’s done before or after you begin selling or distributing it. In copyright law language, selling or distributing your Prose or CDs is referred to as having ‘published’ your work. (No fancy or formal publishing required. Publishing can be as simple as distributing photocopies of your Prose or distributing CDs burned from your laptop and labeled with magic marker, or self publishing… even distributing copies of your Prose handwritten on napkins counts as publishing.)
One extra step is required to copyright work that has already been distributed or sold (ie published). If the work you are registering for copyright has already been published before you complete the online copyright registration process, then you will be required to mail in 2 copies of your work (ie your CD, book, Prose, photo etc.) to the copyright office. The online copyright registration makes it simple by providing an address and printable mailing label to attach to your package.
Watching the tutorial provided by the US Copyright Office on how to complete the online registration process (eCO Tutorial) before you start your registration helps.