The popular distribution channels include:
- Apple iTunes Connect [accepts ePUB and PDF/A for distribution to iPad and iPhone through iTunes]<https://itunesconnect.apple.com/WebObjects/iTunesConnect.woa/wa/apply>
You need to apply for a publisher relationship with the above channels and create an account with each one of them.
Before you start uploading the content files to respective channels, please keep some title metadata handy:-
Description, Subject Category [BISAC <http://www.bisg.org/what-we-do-0-136-bisac-subject-headings-list-major-subjects.php>], Keywords, ISBNs, if any [mandatory for Apple iTunes; not required for B&N and KDP], Book Reviews, Author Biodata, and Price.