The EPUB format has become the widely accepted standard for ebooks. Every non-dedicated e-reading device out there (cell phones and tablets) have apps that can read this format, whether we are talking Stanza for the IPhone or Aldiko for Android. Additionally, Barnes and Noble's Nook uses a modified version of the EPUB format, which is why I suggest using an EPUB file to upload your book to their PubIt service. It can take other formats, but if you're going to create an EPUB, it makes sense to upload using their native format.
To manually create an EPUB file, however, is not an easy task. The EPUB file is actually a zipped file containing several files. There are some control files that "direct traffic" so to speak, artwork files for any graphics used, and the text in an html format divided up by any needed page breaks. But luckily there are some programs that will automate this process, even free ones, that will make quick work of the conversion process.