If it's the first time for you in Paris... don't take RER B Line from CDG to Paris.
1. As a French who wants to welcome you

the "view" from the train along the trip is not the best... even if I know every big city has its "bad" area...
2. If I was a tourist, I think this train line is not adapted... Signs are not clear, and for instance in summer this line is closed (for refurbishment ... and it needs it...) after 10pm and it's notified nowhere !!!
3. Beware the signs because some trains are express (direct to Paris 1st station), and some stops every "typical..." towns of the north suburb... (known for some incidents) and trains can get very overcrowded if you have luggage with you.
If you can, try to find the "Roissybus" shuttle. It's direct from Paris CDG to Paris Opera station (metro line 8) by freeway every 15 min. 8,30€ single trip I think, as a RER trip is.. And this line is generally used by tourists, it's very safe

And vocal informations are provided in english in the bus.
Ohhh and i forgot... Welcome to France!!!! Enjoy
(There is a project to build a new modern CDG-Express train in the next years, after the new automatic metro between all terminals will open next year)