I understand that one popular starting place is the Cathedral of Notre Dame. However, I cannot personally confirm the presence of signs, arrows or conch shells (in the pavement or on signposts) marking the route from there. This is simply because my last visit to the Cathedral predates my Camino experiences. So, I was not looking for these directional clues.
That said, back in medieval times, this would have been a logical starting place for pilgrims from Paris. And the cathedral was certainly there.
Others with more specific information, please contribute if you can.
I hope this helps.