I am wanting to write a document to become a ebook/epub.

What program can I use? Or how can I get started? I want to be able to create chapter headings that one can go to on an ereader.
Can anyone give me some direction. Note: you are talking to a person who does not know a whole lot of computer language (and is feeling quite lost as I read a lot of posts on this forum), so, as I would truly appreciate some guidance, please go gentle on computer lingo.