How did the Incubators start making Magical Girls? The deal the Incubators make to girls is that by granting one wish that a girl desires they become a Magical Girl and fight Witches.

We know that Witches are in fact Magical Girls, so I am wondering how did the Incubators first start making the Magical Girls when there were no Witches to fight (as there were no Magical Girls to become Witches)?
Witches are not needed for the Incubators. Witches are needed for the survival of the magical girls. The Incubators could not care less about what happens to the magical girls as long as there are enough girls to turn into Puella Magis.

The sole purpose of the Incubators making Puella Magis is to generate energy. There would be no need to have witches. 

Of course, there is another explanation. Familiars can form from Puella Magis before they turn into a witch. Since Familiars can become witches themselves, it's possible for the first Puella Magis to encounter witches.