It may seem obvious but you definatly need to bring a valid driving licence plus it needs to have been held for at least 1 year and be in Western Script. For British license holders you should supply both the card and paper versions at the rental desk. If you require 2 drivers for the car rental then both drivers must have licences with them at the time of pick-up, the main driver should also have the credit card in thier name.
A credit card, as oposed to a debit card, is required when hiring a car in France, this credit card is tested for the deposit amount for the car, a debit card cannot do this, cash is also not accepted for this purpose, niehter is a cheque.
Although these are available with some rental cars, they are not supplied with all cars and will be supplied in French language (this maybe possible to change but not easy). If you have a Sat Nav / GPS of your own then consider bringing it with you, it may well be cheaper to even buy a GPS before leaving home, but please make sure France is installed via your map software.
Although road maps can be found in bookstores and in all service stations. When you arrive at the airport you will probably need one straight away (if you do not have GPS) so consider buying a map before you leave on your journey.
It is a good idea to have some cash on you, if you arrive on a Sunday you may find that the fuel station is shut and will not accept your credit card (this is a sure thing if you have the old style credit card i.e not chip and pin). If you have cash with you then you can always sweet talk a frenchmen into buying the fuel for you and paying them in cash. It happens quite often!
Check your mobile phone has roaming enabled, i.e it will work abroad, also check that data roaming is turned off, or at least be aware that data roaming in a foriegn country may inncur significant extra charges.