The first thing you want to do is get into contact with a good accountant (Expert comptable).

Another source of information would be a notaire or avocat, they will be able to provide useful information about the legal aspects of starting a business.

You will need to contact the Chambre de Commerce et d’Industrie (CIC) deal with regisration of busineses in France

There are some good guides for this around on IFP, Okoshken and leadingineurope

Regards,

Richard