You can take a private shuttle to get there. There are many, many listings of private shuttles on TA.
It appears to me (please correct if I'm wrong) that the least expensive, most direct public transportation option for you would be
Roissy-bus, which run every 15-20 minutes from CDG to the Opera metro stop. Cost is 8.50 euros - pay the driver. You would then walk to the front of the Opera house to get on Metro Line 3 direction Gallieni. You would need to buy a Metro ticket.
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