Here is what I advice:
1. Head over to the site of
Jedisaber and read the introduction about ePUB there. It gives you insight in what an ePUB actually is.
2. Work through the HTML/CSS tutorial from Pablo on this site. I believe in the Workshop or ePUB thread. It should give you basic knowledge, enough to start.
This will not make you an expert, but enough to get started. Read the Sigil manual if you want/need. Dive in and try. Do not make it too complex to start with, start simple. Remember, less is more. The less complex code you use, the better the reading experience on all devices.
If you need to look up stuff, use the W3 pages or read through the ePUB specs at IDPF.
Look at some of the free books here to get you going. Also, stay open minded. You can always learn something new.