The standard HTML output of Word is horrible, the filtered HTML output better, but still not good. It produces very bloated code. Be prepare to clean up a lot of code.
The Sigil and Calibre viewer are basically a webviewer and not representative for a reader.
If you want to use Word, you need to either cleanup the code or use something else to produce clean HTML. I have created a macro for this (it is in the ePUB subforum), but there are other methods.
If you want to create ePUB's, you will have to learn at least the basic (x)html/css. There is also a
tutorial on the site which should get you underway.