Getting from the Charles De Gaulle Airport to central Paris is easy. Look for signboards like above that will lead you to the tourist information counter, where you can purchase train or bus tickets. (Or if you wish, you can buy the RER ticket at Terminal 3).
Then take the CDGVAL (or airport shuttle) to terminal 3, where it connects you to RER B Line and the bus station. In the case of terminal 1, the CDGVAL is located at Hall 5 and 6. (On a side note, if you need to get to TGV station, go to Terminal 2).
Charles de Gaulle and Orly http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Home/