Accessible from the airport at Roissypole, RER B train line goes into central Paris. At Châtelet les Halles station, switch to RER A to Cergy-Préfecture. The fastest way to Cergy is to use bus line 95-18 which links Roissypole to Cergy-Pontoise in 1 hour, for 7 €, from 6 a.m. to 8 p.m.
To reach Paris, your trip will be considerably shortened if you use the Air France shuttle buses that bring you into the heart of the capital in 30 minutes.