1. RER A
If you take the train to go to Cergy, you should take RER A West towards Cergy-le-Haut and get off at Cergy Prefecture station.
Consult Transilien website or use the The Île-de-France Mobilités app
2. Bus 95-18
If you don't have much luggage, you can also take the bus directly from the Charles de Gaulle airport for 2.5€ (please have the exact amount since the driver cannot give you back any change) and approximately 1 hour later, you are in Cergy-Préfecture. The bus stop is within a 10/15-minute walk from the campus. The bus runs regularly from 5:20 am to 11:00 pm during the week and from 6:21 am to 9:21 pm on weekends. See the PDF the full schedule in attachment.
How to find the bus station and the bus at the airport?
Go to Terminal 3 and to the Roissypôle bus station, Platform G1:
3. If you prefer to hire a taxi, see this page