Unless on AF you will arrive Terminal 1. Take the bus to the train station in the Terminal 2 complex. There is a regional train into the city. Link to CDG map:
http://www.paris-cdg.com/maps.html

Train Services
The rapid RER train service links Charles de Gaulle Airport with central Paris. Trains run every 15 minutes (eight minutes in peak periods) and the journey takes approximately 35 minutes. Line B runs from the TGV station at Terminal 2 to Gare du Nord, Châtelet-les-Halles, Saint-Michel and Denfert-Rochereau, with connections to the metro.

The three terminals are connected by free shuttle buses. Terminal 1 handles international flights, excluding Air France, Terminal 2 Air France and other airlines flying to European destinations.