Translation:According to him, this dress is black.
It took me years to learn the difference, but suknia/sukienka is the dress (starting from the arms, usually) and spódnica/spódniczka is the skirt (starting from the waist).
I think suknia is actually a gown. but what is a difference between those two I do not know.
"według X" = "according to X", "in X's opinion/view", stuff like that.
It doesn't just mean "an opinion".
When to use Jego/Niego/Go? Are they completely the same when they are in the same case?
OK, so all of them are either Genitive or Accusative. But no, they're not the same.
The basic, most neutral form is 'go'. "Lubię go", "Potrzebuję go".
Then you have the more emphatic "jego", usually showing some contrast. "Lubię jego, a nie ciebie!" "Potrzebuję jego, a nie was!".
And then there's "niego", an example of what I call "n-forms". The n-forms, if such a variant exists for the given pronoun, are used only (and they're obligatory then) after a preposition. "Idę do niego". "Liczę na niego".
And of course "jego" means "his" in all cases, but that's a different thing.