Hope you soon get some more detailed answer about the best provider and cheapest rates available because it's already some time ago that I used this and such things are changing basically all the time.
I went into a phone shop in the city and asked what prepaid packages are available. I got the network Orange and this package was called "Mobicarte Prepayée". It was a little box, like a dvd box, and there was the sim card and a few instructions. It worked with my GSM Siemens phone without problems. International calls to Germany or Austria were around 0.70 € for me, receiving calls for free. After a few months your number gets deleted if you don't upload more money.
But this is already some time ago, you certainly need more infos from French people as a lot could have changed since then and that you get better informations about what you need.