(2-part post)
It's easy.
My preference is to travel by rail. That means taking the free CDGVAL automatic shuttle service to Terminal 1. (if you accidentally go the wrong way, and end up in Terminal 2, it won't matter. The RER runs from there as well). Then just follow the 'RER' signs. There's only one line (Line B) serving the airport and you're at the end of the line, so it's impossible to go the wrong way. The fare into central Paris is �8.20 but, if you're going to be making more journeys on the RER, Metro or bus services, it would make better sense to buy a 6-zone Paris Visite ticket. Fares here:
http://www.ratp.info/informer/anglais/paris_vi site.php
RER Line B offers good connections, by RER or Netro, to all areas of central Paris. It takes about 30 minutes to get to key interchange stations like Ch�telet Les Halles or St-Michel Notre-Dame. (Use the links, down the left of that link above, for RER and Metro maps).
http://www.aeroportsdeparis.fr/Adp/en-GB/Passa gers/Arrivees/QuitterAeroport/EnTransportEnCom mun/quitter-Paris-CDg-RER-B.htm