can you tell me were online can I purchase a train ticket from CDG airport to Denfert Rochereau and can I purchase a metro ticket online also and were.

The bottom of this article has links to several online ticket sales for Paris city train transportation.  This website is the city public transit authority, but they do not sell tickets directly online, only through partners linked on that page.
 
https://www.ratp.fr/en/titres-et-tarifs/paris-visite-travel-pass