You can consult the metro and RER timetables on the RATP website and download the metro and RER map
From the Roissy Charles de Gaulle airport:
You can take the RER B line. It takes about 45 minutes from the airport to the center of Paris. It stops in many stations within Paris, including Denfert Rochereau and Port Royal, and allows connection to metro lines. There is one RER B every 10-15 minutes. The price is 9,25 euros. Please note that the RER B stops in two stations within the Roissy Charles de Gaulle airport: CDG 1 and CDG 2. Check your plane ticket to know to which station to go for your return flight. See the information on RER B.
Alternatively, you can take the Roissybus, which brings you to the Opera station in 45 to 60 minutes. There is one Roissybus every 15-20 minutes. The price is 8.90 euros. See the information on Roissybus.
Finally, you can take the Air France buses (even if you are not travelling with Air France). Line 4 brings you to Montparnasse in about 1h15 minutes. There is one bus every 30 minutes. The price is 17 euros. See the information on Air France buses.
From the Orly airport:
You can take the Orlyval automatic metro to Antony station. There is one metro every 4-7 minutes and it takes 8 minutes. In Antony change to the RER B line towards Paris. It takes about 15 minutes to reach the center of Paris, it stops in many station and allows connection to metro lines. The price is 10.75 euros. Please note that the Orlyval stops in two stations within the Orly airport: Orly Sud and Orly Ouest. Check your plane ticket to know to which station to go for your return flight. See the information on Orlyval.
Alternatively, you can take the Orlybus which brings you to the Denfert-Rochereau station in 30 minutes. There is one Orlybus every 15-20 minutes. The price is 7 euros. See the information on Orlybus.
Finally, you can take the Air France buses (even if you are not travelling with Air France). Line 1 brings you to Montparnasse in about 35 minutes. There is one bus every 30 minutes. The price is 12 euros. See the information on Air France buses.
Paris offers 4 train stations: Gare du Nord, Gare de l’Est, Gare de Lyon and Gare Montparnasse. All four stations are within Paris and well connected to the metro and RER system. You can check the train timetables on the SNCF website