While searching on the Internet, you will find info about a special Paris metro pass dedicated to tourists, allowing you to visit Paris and the close suburbs for short periods of time, from 1 day up to 5 days. This pass is called the Paris Visit Pass, or Paris Visite.

Though this pass is very expensive, and usually not worth it...

Paris Visite Pass fares:

1 DAY   - 9.30 E
2 DAYS - 15.20 €
3 DAYS - 20.70 E
5 DAYS - 29.90 E
 

In my opinion, the Paris Visite pass is a tourist trap.

You would have to spend half of your day in the metro to make it worth it :-)

A bunch of 10 tickets costs less than 12 €. 

You can purchase bunch of tickets at any metro stop: there is a desk with an employee (more or less fluent in English :-)

or you can use a small machine looking like an ATM: you can select the langage, and after a few easy steps you can pay with a credit card to get your bunch of 10 tickets!

Furthermore, Paris is a very walkable city and you will only need to take the metro to cover long distances within Paris.