I was told that it could be that I have no right for the german citicenship
Eh, Elm, if u're a German citizen by birth then u should still hold German citizenship- unless otherwise!
Renew your German passport first if u're thinking of going for the Polish one.
The procedure of acquiring Polish Citizenship through your Parents, as many have found out on here, is quite long and complicated.
I'm sure u'll need tons and tons of documents to prove who your parents were.
There are many useful posts on here.