Sounds as if you want to produce a Reflowable Text ePub. This is fairly easy to do with InDesign CC2015 if you're used to producing documents with the application. But it's not intuitive and if you've never produced these before I strongly advise you to get some help. Lynda.com have an excellent online tutorial: InDesign CC 2015: EPUB Fundamentals | Lynda.com
You can get 10 days free access to try it out.
(I have no connection with Lynda.com!)
This is a duplicate post. I'm locking this one.