My first time in Paris, best transportation to Holiday Inn Gare Montparnas from CDG in the morning.
take an official taxi if you have 3 people; it should cost more or less 50 euros flat rate
or take Air France bus to Gare Montparnasse http://www.lebusdirect.com/
The flat rate for up to four people is 55€. Best is hard to say. Easiest is a taxi. You could also take Le Bus Direct. See http://www.lebusdirect.com/. You would need Line 4. You don't say how many are in your party or how much luggage you have or if you are on a tight budget so it's hard to say what would be best.
If you are unfamiliar with public transportation, it might be easier to take a taxi, especially if you have luggage.
Just follow the overhead signs in CDG to the TAXI stand. Remember that legitimate taxi drivers always remain in their taxi.
It would be a good idea to print out the name and complete address of your hotel, including the Postal Code, so there won't be any confusion.
If you need to use a credit card, ask before getting into the taxi - some take them, others don't.