Hi Barbara,
There aren't any direct trains from CDG to Caen, but you can first go to Gare St. Lazare (station) in Paris, which has a train direct to Caen.
To get from CDG to Gare St. Lazare station in Paris, first take the
RER B from CDG to Paris getting off at
Gare du Nord station, then transfer to the RER E direction St. Lazare, using the same
Billet Ile-de-France (ticket).
The Paris - St. Lazare station to Caen is currently serviced by Corail (brand of) train, which is not high speed. Paris - Caen by Corail takes 1h47 minutes. Train tickets are about 30€ when purchased ahead of time, 45€ when purchased last minute. To find out more about buying train tickets ahead of time from outside of France, see the
Buying TGV tickets article (also applies to non-TGV trains).
Best regards,
Ben