I wrote a whole book on how to create EPUB files that you can upload to Apple's iBookstore, but what do you do once the EPUB is complete?
Apple announced in May that they were opening up the iBookstore to anyone who could satisfy certain requirements, including, but not limited to having a Intel-based Mac, having an ISBN for the book you want to distribute, having an EPUB file, having a US Tax ID, and finally, having a valid iTunes Store account.
You have to apply to Apple to be accepted as a publisher in the iBookstore. To do so, start at iTunes Connect. They will explain all of the requirements in more detail, and once you are accepted, you will be able to download iTunes Producer, which is the program you will use to upload your EPUB files to Apple. You can choose to apply DRM to your titles, or you can leave them DRM free.
But, contrary to an article I read today, Apple does accept your EPUB files as is, there is no conversion required or applied.
And, for those of you outside the US, don't forget that you can create and sell EPUBs directly from your own sites. It's not a perfect solution, but people can buy and download your books and then upload them to their iPads.