There is a direct train from Gare Saint-Charles, Marseille to CDG terminal 2 (Aéroport Paris - Roissy - Charles-de-Gaulle (CDG 2) - RER), you dont need to do nothing just jump to train sit relax and in 3:50 you are in CDG airport terminal 2.
If you want to see dates etc. visit
www.sncf.com and you can plan your journey and even buy your ticket online.