The website easycdg may help you figure out where you are going to land if this is your first time in France.
Or you may consult the official website of Paris airports
When you travel lightly (one wheel suitcase) and are not exhausted by a long flight, you may consider riding the 9518 bus to and from Roissy CDG airport. At the airport, look for the sign CDGVAL that will lead you to the bus station.
The ticket costs 2.50 euro, be ready to have some change.
From Cergy Prefecture bus/RER station to ESSEC, there will be a 10-mn walk.