Or, if you are tired from your flight and you want to rest for a while, take the free-standing elevator at the corner of Avenue du Maine to the top level. It leads to a park built above the train station. From inside the station there is a long stairway leading up to the park, at the far left of the train platforms. Either way, follow the signs to the ticket windows. Remember that trains are very punctual in Europe.
Second class is probably plenty comfortable.
You can choose smoking or non-smoking sections. Here are some typical times for trains to Chartres from Montparnasse. There are many more, depending on which day of the week. You will get to Montparnasse about 50 minutes after leaving Charles de Gaulle Airport. Suppose you are hungry and you want to get a bite to eat before going on to Chartres.