I've come across this useful online tool enabling would be entrepreneurs to calculate how much social charges they'll pay, according to their business type. Your social charges vary according to your business structure (entreprise individuelle, EURL, SARL) and business regime (auto entrepreneur, micro, regime reel simplifie).
How much tax will I pay in France is one of the most frequently asked questions. It is also difficult to give a clear answer, since it vary according to your business structure, as well as your estimated turnover and expenses. So have a look at the comparateur de charges sociales, it will give you a good idea of what you will pay.