Just take the RER B to city center. Than, you may pass to the Metro line by buying another metro card.
Or you may just buy a Paris one-day visite pass including the zones 1 to 5 that will enable you to use both the RER B and the metro. You can buy this card for the first and the last day of your trip, and on the others you may pay each ride a single amount (around 1.90 euro) or buy a day pass for zone 1 to 3 (for around 10 euros).
And to take the RER B you can follow the documents below by downloading them, or just see the maps below.
The connection between terminals in CDG Airport:
CDG Airport map: