The RER train service (Line B) will take you into central Paris and offers direct connections to the Metro, so that you can get to your final destination. For directions to the train, from each of the terminals, see here:
http://www.aeroportsdeparis.fr/Adp/en-GB/Passa gers/Arrivees/QuitterAeroport/EnTransportEnCom mun/EnTransportsEnCommun.htm
(The journey time, into the city centre, is around 45 minutes and the fares are very reasonable)
There are also bus services from Terminal 3. (If you arrive at a different terminal, there's a free shuttle bus across to Terminal 3). The express Roissybus runs to Op�ra, taking between 45 and 60 minutes. However, it's more expensive than taking the slower local services (Service 351 to Nation or 350 to Gare de l'Est).
http://www.ratp.info/informer/anglais/aeroport s.php#
Personally, I wouldn't even consider taking the bus. The RER service is fastest and offers the greatest ease of connections with the Metro. (You can buy a through ticket to your final destination).
Chris