Hi Riogusto,
See the
RoissyBus from CDG Airport to Paris article, which will be your easiest way to get from CDG to your Hotel Best Western Montcalm.
Your
hotel is located next to Boucicaut station on Metro Line 8 (Google Map).
The Roissybus, after leaving the airport, stops only at Opera in Paris, which has a station for Metro Line 8. The Opera underground station is located in front of the Opera house. The picture of the Roissybus stop in Paris in the above article shows the bus parked on the left hand side (west) of Paris Opera (the large white building with stairs curving up near the right hand side of the photo). The bus is parked facing towards the front of Opera.
You cannot use the Roissybus ticket (8,90€) for the Metro, thus you need to purchase a
Metro ticket, a ticket t+, at this station. Cost of a Metro ticket is 1,60€. Take Metro Line 8 in direction Balard until you reach station Boucicaut. The Best Western is very close, as visible in the Google map link above.
If you decide to purchase a
transit pass like the Navigo Découverte/Carte Orange, it covers both the Roissybus and the Metro.
Best of luck,
Ben