Check with your school to see if they have bulletin boards or other sources of student housing. Rental agencies require a fee of a month's rent, so you may want to try other avenues first. Depending on your school and what you're studying, there are some associations or other groups that help students find housing. (For example, for science students and researchers in the Saclay area, there is an organization called Science Accueil that works with local landlords to find cheap housing.)
What many people do is to book temporary accommodation on arrival and then find long term housing once they are on site. What are called "student apartments" can be pretty grim and you want to see the place in person before putting any money down.
There are also starting to be several "co-location" (roommate) agencies in Paris and elsewhere in France. Try googling "co-location" and Paris to see what you find. Probably safer to set this sort of thing up when you're on site, so you can meet your potential roommate in person before deciding, but it's becoming a popular option here.