Hi Tina,
You'll need a train ticket for
CDG Airport to Paris and another train ticket for
Paris to Marne La Vallée Disneyland, but you can buy them at the same time at CDG Airport.
Approximate cost will be 15€ and about 1 hour 20 minutes for the trip. Keep in mind that it is also possible to catch a highspeed TGV train from
CDG Airport to Euro Disney by train, which can cost as little as 15€ (average of 25€) and takes only 10 minutes to complete the trip.
Best regards,
Ben