Paris to chartres

The train into Paris is called the RER. You would have to change to the Metro to get to Gare Montparnasse at Denfert Rochereau on the No 6 line. That is included in one ticket for 11.40 EUR per person.
You might want instead, to take Le Bus Direct line 4 with your luggage stored underneath the bus. Cost is 18.50 EUR per person.

At Montparnasse, buy tickets to Chartres.

The metro doesn't go to CDG which is a long way from Paris. Assuming you will catch the train at Gare Montparnasse you have a couple of choices to get there. A cab would cost 55 for 4 people and their luggage and 4 Euro for each additional person up to 7. The RER train from CDG to Paris costs 10.30 a head; you would take the RER B into the city and then change to the metro. The obvious route would be RER B to Denfert Rochereou and then the metro 6 to Montparnasse Bienvenue and then on to the train station. You will be carrying bags up and down stairs on this trip and must be pickpocket proof. It is all well signed. There is also a bus that goes from the airport to Gare Montparnasse, Le Bus Direct that costs 18 a person. The rule of thumb is 4 hours between ETA at CDG and a train in Paris or you could just buy tickets on arrival and not worry about a late plane or slow immigration lines.

Can we just take the metro into Paris and on into Chartres?

Chartres is about 50 miles from Paris (the "Metro" doesn't go that far).

If I were arriving at CDG and wanting to catch the next convenient train from Gare Montparnasse to Chartres, I'd book a ride from the airport with a car service such as Paris Webservices. The year we went to France, they were recommended in the RS guide to France and you got a discount if you showed the driver your copy of the book. They have a system where they'll wait for you if your plane is delayed. Our driver was kind and helpful, spoke good English (had gone to college in the US), and the car was nice and new and clean. Doing it this way is a lot less hassle than wrangling luggage on bus, metro, etc. and having a reserved car feels safer than getting into a cab.

Having a reserved car is not 'safer' than as cabs are always there waiting in the official taxi queue with their fixed price ride into town; car services usually cost more and they are notorious for not showing up. If you use one, at least don't pre-pay. Of course never go with anyone who approaches you asking if you 'need a taxi'; these are all crooks.

I second the others, CDG to Montparnasse is quite the schlep by RER plus metro, with at least two of you I'd take a 55 euro taxi. Once at Montparnasse the trains to Chartres are quite frequent, you'll typically wait less than an hour for the next one.

Thank you for your replies,

It seems taking a cab from the airport to the gar at Montparnasse is the way to go then. Excellent. I’ll have to investigate taking cabs from de Gaulle.

Thank you so much!

kdgermain, a note on terminology. "gare" is French for station. "Gare Montparnasse" is the station that serves destinations south-west of Paris, including Chartres.
As the others say, for a newbie with luggage, the easiest way to get from Charles de Gaulle airport to Gare Montparnasse is a normal taxi at the flat fare of €55. Unless that blows your budget, in which case use RER+métro at €10.30 per person.

As you cannot predict what time you will emerge from the airport (plane could be late depending on winds, immigration could take 30 minutes or 90), it is better not to buy rail tickets in advance, instead buy them when you get to Gare Montparnasse.

You can look up train times on the SNCF (French Railways) website: https://ch.oui.sncf/en/
I see 2-3 departures per hour, it takes just over 1 hour and costs €16.40 per person.

You need a train to Chartres from Paris, it's roughly an hour (average 58 minutes up to 1:15, if it's much longer than an hour, look for an earlier or later train) from Gare Montparnasse. I use the app Next Stop Paris to get the best current metro route. Looking at the map, I'd likely take an RER B from CDG to Gare Nord then connect to Metro Line 4 (but use Next Stop Paris for the best routing).