No, that would be "Das ist meine Sache". We don't use the plural form there :)
I think Sache is often something you can touch. Ding is more a thought. But I am not German, please correct me... :-)
Das can denote These? in tips section it says Die can denote those/ these and der/das can denote this/that... also what about Dies/diese i have never come across these in the lessons but duolingo is using them.. can someone please make the Sachen klar..