I completely understand that this is an area of stress for many people moving abroad, whether as an assistant, on study abroad, or just because. While I am far from an expert, here’s what I’ve learned:
Both years I spent as an assistant in Lille I tried to find roommates, and was unsuccessful. I originally wanted roommates because the thought of living by myself for the first time ever in a country where I barely spoke the language and knew nobody wasn’t very appealing.
However, some other assistants that I know were able to find roommates. They used the website appartager.fr, which is like a roommate searching site. It's not the best because you have to pay to be able to access a lot of the contact info.
I gave up trying to find roommates after about a month and a half of searching. I got myself a nice little studio, and I loved it!
The website that I’ve used the most often and had success with is leboncoin.fr. It is similar to craigslist back home. Every single apartment I have rented in France has been from this website.
In order to use the website, click the area of the map that corresponds with where you are in France. Then in the first drop down menu, select “location” for apartments, or “collocation” for roommates. Then it’s possible to make the search more precise, with options like highest rent/lowest rent, number of rooms, furnished, etc.
There are numerous rental agencies all over France. Some even place ads on the leboncoin (it says professionel). With them, just be prepared to pay them the equivalent of the 1st month's rent (if not more) after they find an apartment you like.
Another possibility would be to live in a dorm like situation. CROUS is France's version of student housing. I tried to get housing with them when I came over here to do my masters, but the registration for student housing is in April, and I didn’t even know I was accepted until June. I was unable to get student housing, but am quite glad actually! I love living in the city center and having my own apartment.
There are also foyers, which are like really tiny studios, but usually come with a shower, toilet, and kitchenette - meaning stove tops and a mini fridge. Some foyers have rules (one assistant wasn't allowed to have over night visitors, or boys in the foyer after 10 - it was an all girl foyer), but some are chill.
On a quick safety note, I wouldn't accept living somewhere until you actually see it. I've seen some pretty scary apartments that seemed good on leboncoin. Also be aware of fake ads trying to take your money, etc.
I've lived in France for three and a half years in three different apartments in two cities, and it's all been an adventure. Good luck on finding housing in France!