There is no tutorial that will explain those errors for you. You just need to learn how to decipher them... with the help of people who've run into them and fixed them before.
Flight Crew (which I believe is the validation engine Sigil uses) gives clearer (
less cryptic) responses, IMHO. Also pay attention to the line number/file name given. That will help locate the problem area.
Some are fairly self-explanatory... like:
"attribute "caption" is not declared for element ´img´"
Somewhere (hopefully the filename/line number narrows it down), an img tag has a 'caption="blah, blah"' attribute. Get rid of that. It doesn't belong.
"ID value "heading_id_13" is not unique"
Element IDs (id="something") must be unique for that entire (x)html document. Change them so they are each unique, or delete them if you won't be linking to those elements (including NCX entries) or applying special css to them based on that ID.