If you don’t pay monthly towards your income tax, then you have probably received a tax demand through the post. Basically the French government is making people pay their 2014 income tax in three installements. They are basing the calculations on the 2013 income tax demand. Don’t worry, if they have overcalculated, then you will be reimbursed any excess paid. If they’ve undercalculated, you will be asked to pay the extra.
This is obviously making people very unhappy, as it feels like they’ve only just paid for 2013. On the positive side, you’ll be paying less out at a time when the tax d’habitation and taxe foncière are also due.