France is actually super cheap for mobile data - if you use it for more than a few weeks, and you shop around a bit (the market is quite competitive).
Types of plans
The most common type of plan is a "Forfait Sans Engagement". This is a 30-day contract including a set amount of calls, texts and data, that renews automatically, which you can cancel at any time. Other types of plan such as pre-pay (top up and then use), and year-long "locked-in" contracts are much less common, and are often not as good value.
Additionally, some plans are marketed as "Bloqués", either when initially signing up, or as an option that you can toggle at any time. "Bloqué" means that you cannot be charged extra beyond the base amount, meaning that things like premium-rate or international calls are not possible, instead of being charged extra as would normally happen.
The big three have roughly equal network coverage and quality, with Orange being the best. Free is somewhat behind in coverage.
Bands are pretty standard European ones, check here for the ones used, and cross-reference with your phone's specifications to see how well it'll work. Slightly less accurate is to check on https://willmyphonework.net/. If you go with Free, a phone which supports LTE B28 (700mhz) is definitely recommended, as that will improve your indoor 4G coverage.
Finding a provider
There are 4 network operators in France, namely Orange, Bouygues Telecom, SFR, and Free. Some of these have their own low-cost "sub-brands", however, all others (technically called MVNOs) run "on top of" these networks, but have their own offers and pricing.
Orange, SFR, and Bouygues (as well as their low-cost sub-brands) require a French bank account (IBAN number), so are useless unless you have one. This Leaves you with Free and the smaller ones.
To get the best deals, you have to order your SIM card online to a french address, however OK deals can also be found by purchasing a SIM card in a big retailer chain store (e.g E. Leclerc).
SIM card registration is compulsory in France, and you have to give the provider your ID.
For actually finding a deal, I've found this comparison page the most useful. (remember, B & You, Red by SFR, and Sosh all require a French bank account). For Canadians who see the that you can get 100GB for €10 a month, ...well I feel sad for you. Come live in France :-)
It lists the promo price (which goes back to the regular price after twelve months, but remember you can cancel any time), the additional data you get while roaming in Europe (there are limits to this though, you can't take a SIM card out of France "permanently"), included calls & text, and promo code to use.
When you order, there is always a "fiche information standardisée" available, which lists all the details and conditions of the offer (cancelling it, use outside of France...), on an easy to understand way (if you can read French, that is.)
These are offers that are usually only available online.
You can cancel these plans at any time, but you always have to do so a specific number of days before the roll-over date into the next month. Check first.
Cancelling procedures range from easy (click a button in your online account), to archaic (send a registered letter).
You can also port your number easily (which cancels the contract automatically), if you port it to a low-cost prepay carrier, you can keep it for very little money if you want to return to France.