Hi Noleafuk,
You're correct: directly from Charles de Gaulle Airport you can catch a
RER B train from CDG to Paris, which will stop at St. Michel-Notre Dame station.
The train station you'll want to use is located in the Roissypole building, a 5 minute walk from CDG Terminal 3. The
CDG Terminal 1 to Paris photo tour article has photos of the Roissypole building and the train station, named Aeroport CDG 1.
Travel time between Aeroport CDG 1 (the train station, not the terminal) and St. Michel-Notre Dame station is about 35-38 minutes.
If I may suggest, the following two articles should give you a good understanding of how the Paris Metro and Paris RER trains work:
Paris Metro and
Paris RER.
Best regards,
Ben