Hi Rama,
The article on
CDG Airport to Paris by RER B train will help you get into Paris. Take the RER B until Saint Michel Notre Dame station. Then transfer to RER C direction Pointoise until station Epinay-sur-Seine.
If you're arriving near the beginning of the week, I'd recommend a 1-5 zone
Pass Navigo Découverte which will cover you from
CDG Airport to Paris, to Epinay-sur-Seine and your multiple trips back and forth from Epinay-sur-Seine to Paris. Plus, if you wanted to
visit Versailles from Paris one day, the pass would cover you for that as well.
Best regards,
Ben