Hi Alda,
A really good start is to get a copy of ePub straight to the point recommended to you on the first thread I read.
http://www.elizabethcastro.com/epub/
It explains so much more than anyone can do on here. Between that book and the help you get here I landed a job writing ebooks the day after I wondered what they were when I saw an ad for an ebook maker in the paper.
An ePub is just a mini website in a zip file. Instead of an index page that points to everything it has OPF file that points to everything, lists all of it's components and stitches it all together. The key is to learn how that file does that and make sure you add the bits that indesign doesn't. I.e. It doesn't add a publish date in the meta data or set up the cover image so it displays correctly. I'm pretty certain that 3 is similar to 4 when exporting to ePub which works great.
You will also need to read the w3 HTML and css "how too" explanations.
http://www.w3schools.com/css/ http://www.w3schools.com/html/default.asp
Fortunately I had done some HTML back in the 90's when sites were new and exciting. I feel the same way now learning something new. You will need to know what HTML and CSS is even if you don't actually learn it. I've played with Sigil and as good as it is I think the indesign export when done properly is by far the easiest option if you already own the program.
I didn't see your location. I'm in Sydney. I'm not sure of your time zone but I'll keep an ear to the ground for you mate.