Finding accommodation in Paris is very, very difficult. Having lived here for a while now and found not one, but two places to live I would thoroughly recommend the following if you're looking for a place in the city, whether it be a room in a flat or a studio.

Tips on Finding Accommodation

Do your research

• If you have never been to Paris before or haven't done your research, you will need to so that you don't end up living in a dodgy area due to an attractive asking price. Whenever I found a flat that I liked I sent the location to one of my Parisian friends to ask about the area just to make sure. Only live somewhere where you will feel safe. 
• Work out where your work/university is and see which metro lines your flat has to be near, unless it's in walking distance of course! In Paris the metro can take a long time and although something may look near on the map it can take a long time to get there if there isn't a direct metro. Check this on Google Maps or on an app like City Mapper before you choose a place.

Where to Find a Flat 

Appartager

For flat sharing - I actually found my flat through here and I ended up in a super cute apartment (the one in these photos), for a reasonable price with the nicest flatmate. Unfortunately to reply to some ads you have to pay a fee (around 30 euros, depending on how long you want the description for), and a lot of the flats aren't that nice but it's worth it. I found both my flat in Florence and my flat in Paris on this website - it exists for loads of countries so even if you're not looking in France you may be able to use it, just scroll down and change the country.
Site: http://www.appartager.com/

La Carte des Colocs

Also for flat sharing - You will have to make an account, and message a lot of people in order to get a response, but at least it's 100% free!
Site: https://www.lacartedescolocs.fr/accueil

Paris Attitude

It's generally a little more expensive and you'll have to pay agency fees but they have some pretty beautiful flats and I know a couple of people who have used it and are happy.
Site: http://www.parisattitude.com/

The American Church of Paris

This sounds really random but it's a known place to find rooms or flats in the city. There's a little noticeboard and you'll have to call or email the owners for more information, but generally the flats are very nice and very reasonably priced - my friend got a studio in the 7th arrondissement (a very posh area, close to the Musée d'Orsay) for 650 euros!
Location: 65 Quai d'Orsay, 75007 Paris

Ask Around

If you're like me and a student on their year abroad, I would highly recommend getting in touch with those in the year above you to see if you can take over their flats once they leave. One of my friends did this and she found the cutest studio in Montmartre which was relatively well priced. It may seem like an effort but you never know what you may find - plus at least you'll know that it's 100% reliable!

Facebook Groups

As with most things, you'll have to be careful that you don't get lured into a scam (as with anything online), but Facebook is a great way for people to find roommates etc.

The following are pretty useful:

Colocation Paris: https://www.facebook.com/ColocationParis/?fref=ts
Colocation Paris: https://www.facebook.com/Colocation-Paris-200189713359265/?fref=ts
PARIS: Location appartement, Colocation, Sous-location: https://www.facebook.com/groups/334192306773040/
Colocation à Paris: https://www.facebook.com/groups/Colocation.in.Paris/
Paris : location d'appart, échange, colocation: https://www.facebook.com/groups/colocationparis/
Plan Appart Paris : Location appartement, Colocation: https://www.facebook.com/groups/planappart/
Plan Coloc Paris : Colocation: https://www.facebook.com/groups/plancoloc/

Erasmus groups and expat groups are useful too - just search for them and a bunch will come up.

Airbnb

This is probably less of a popular choice due to the price, however sometimes people advertise their flats on there for short periods of time (aka 6 months) for them to be sublet - if you're desperate you can always rent one for a couple of weeks for when you get settled in so that you can property hunt once you've arrived.
Site: https://www.airbnb.com/

Most importantly don't give up, new flats and rooms come online every day so keep persevering and you'll find something. Good luck!