Milanya already wrote perfect corrections. I just want to write you translations, so you might understand it a little better

Я у никого не был.
It translates as: I haven't visited a person called nobody
Он с никем не говорит.
He is not speaking with a nobody
Я не обсуждал это с никем.
I haven’t discussed this with a nobody.
Я бы никогда не сказал такую вещь о ником.
I would never say such a thing about a nobody.