Today, in this post, I’m collecting some useful information about Paris (information that I’ve collected during my trip).
From Orly Airport to the city center:  I chose the OrlyBus (ticket 7,40€). It stops at South and west terminals and then it goes straight to the city, stopping at Denfert-Rochereau (from there you can take the underground or RER). The alternative is a shuttle that takes you to the RER stop.
Metro tickets: for a single ride the ticket costs 1.80€. To save some money you can buy a carnet of 10 tickets for 13€ (5€ saved) and you can buy it from the vending machines (at the counters don’t take cash).
Metro lines: the underground is identified by an “M”, a number. The underground in Paris works like the London underground: all depends from the direction of the line you have to take (north – south or west – east); once you have understood the direction you have to travel to all will be very easy!. Surely it is easier to do that to figure out on the basis of a text  written on the internet!
RER: each line is identified by a color a letter. It works similarly to the underground, but in some station isn’t so easy to understand which platform you have to go to: I suggest you to ask someone to be sure ;).
Food: Paris is not a cheap city and you have to keep in mind that even a basic food like pizza may cost up 10€ or 11€ (in Italy, normally, it costs between 5€ and 8€). Kebab is the cheapest food I’ve found.
Museums: If you are between 18 and 25 years old you can enter for free, or with a (good) discount in almost every museum of Paris! You have to show your document to the counter and to the ticket controls and you can enter and enjoy your visit. The musée d’orsay is one of the museum where you can enter for free.