There is a monthly pass you can put on a Navigo Decouverte card ( which costs 5 Euro and is good for 10 years.) The monthly pass is about 85 Euros -- and covers all public transport, trams, buses, metro and RER local trains in the ile de France i.e. all 5 zones in the Paris area. It runs the calendar month so if you are there for a few days in September or November, you will need other tickets or passes for those times.
The card can be bought at any metro and you will need head shots 25/30mm -- you attach the picture and print your name before sealing the card. You can load weeky or monthly passes and day passes on the card. for individual. metro or RER tickets you need a different product.