It is not currently known which god the Greeks were worshiping first, however worship of many of the Olympian pantheon predates the written word in Greece, and the myths themselves relate to Greece's prehistoric past.
Now mythologically, the first "god" was Chaos. From her all other gods in the Greek pantheon draw their lineage.
Primeval 'matter' without any characteristics that everything evolved of
They were born of older primeval gods and goddesses of creation.
Minotaurs do not exist, it was a creature from Greek mythology
In Greek Mythology, yes.
No, it is a creature in Greek mythology
No one; the individual does not exist in Greek myth.
No.
Janus did not exist in ancient greek mythology.
Greek Mythology pre-dates Noah and the tales of the Bible; so, yes, it did, so too did Egyptian Mythology.
The ancient primeval gods (first-born elemental gods) or "Protogenoi" of Greek mythology were the basic components of the universe which were emerged at creation; of which Nyx was counted among.
Chaos was the first primeval goddess to appear at the creation of the universe.
Yes, they are a part of Greek beliefs, and are in Greek stories, but that does not mean they do not exist.