How to get a PhD (doctorat) in France?
As somebody who's actually doing a PhD in France, maybe I can provide some more information.

Firstly, as everybody says, you have to get an equivalent of a Bachelor degree (I think that's "licence" in France), and the a Master degree as well. I didn't do this part of my studies in France, but, some things that I overheard:


of course, nobody is paying while you are doing that, and you most probably have to pay a yearly tuition (probably around 400eur)
I think that there is some programs to get some scholarships that could help you, and you'll probably be eligible to help from CAF for a while
to supplement your income, I'm sure you can teach already in the last year of Master, and possibly (but I'm not sure), even as soon as you finish you Bachelors (aka "licence")
other major expenses are living costs and eating costs: you can usually get a student dormitory (200-300 eur/month is plausible, but it depends on a city) and as a student, you get cheap student lunches (3-4eur/meal)


As I said, I didn't do this Bachelor-Master part of my education in France. But, for the PhD I can offer more information:


a PhD in France is basically a normal work contract lasting for (exactly) 3 years, with a usual possibility of extending it for 3-6 months
to obtain a PhD, you have to obtain a grant. Sometimes, you apply to professors for a subject that is already funded, and sometimes you apply to the professors and then you submit your grant application together (like me)
you still have to pay a yearly tuition (~400eur), but you will be receiving a monthly salary where the amount depends on your grant. It's usually between 1300 and 1600eur
in your first year, you are still possibly eligible for CAF
there's two type of PhD contracts: research, or teaching. The teaching contracts are harder to obtain, but it is usually easier in your second year than your first. If you need/want more income per month, a teaching contract gets you an additional 300eur/month on your salary
other major expenses are again lodging and food: again, sometimes dormitory housing is available, but renting a flat can be acceptable as well (around 300eur/month if your flat-sharing up to 550-600 if you're renting alone). Usually, the lab/university provides some kind of lunch discount tickets as well.




As for some additional information about how to get in to a PhD programme:


in France, it is obligatory to do at least one internship in a research lab or a company during your Masters, but two are possible
if you are planning to stay in France, it is very customary for people to do their PhDs in the same lab where they did their internships, and some even just extend on their internship topics
your Master supervisor and all the contacts you made during your internships will probably be most helpful, as you can ask them for advice, or even if they know of a position / professor that would suit your plans
additionally, if you want to do your PhD outside of France (it always needs a Master degree), it's probably a bit harder to obtain (although your Master contacts can help you). It's a paying job with a salary in a lot of European countries (some examples includ: Germany, Italy, Sweden, Switzerland...)