Rents in Paris are very high. Usually it is cheapest to rent a room in a shared flat.
Airbnb offers an initial overview of the housing market and monthly rates.
A room costs on average € 1,000, but it’s possible to find one from € 500.
The following sites are good options for finding a room in a shared flat:

• appartager (brief registration necessary)
• recherche-colocation
• entre particuliers

It is, of course, also possible to rent your own flat. Landlords usually place their advertisements here. If you want to look at a flat you must always arrive an hour before the time indicated in the advert.

It’s also possible to find rooms in Paris on wg-gesucht.