Going from CDG to St Lazare to Caen to get rental car for first time to France

Just take a taxi from the airport official stand for 55 euros if this is not too much. It is difficult for a first timer to France to take the RER B especially if you are not use to public transportation in big cities. The RER B can get really crowded with everyone since it is only around 14 euros, which includes a transfer to the metro that would take you to Gare St Lazare. Spend a little more to avoid crowds and COVID and professional thieves that can spot a tourist a mile away on the Paris metro

I see you are from West Chester, PA. If you are comfortable taking SEPTA, you can take the RER B.

The taxi fare is 53€. I am big believer in taking a taxi in your situation but the train is not difficult if you are willing to make a transfer and follow signs. But if you are opposed to a taxi or the train, consider taking the Roissy Bus from CDG to Paris and walking 10 minutes to the train station. That about the same amount of walking (maybe less) that you would have to do to take the train and of course, being a bus, the trip will be a bit longer than the trip by train. Other than a taxi, it is my preferred way to get from CDG to St-Lazare.

Are you flying into CDG from the US? If so not driving from CDG to Caen is a good idea and not just because of the rental car savings.

If you decide to travel by mass transit, you should download CityMapper or the RATP's app for transit directions. There are several ways to get to Gare St-Lazare from CDG via the RER B and any connecting line or the RER B may be having service issues when you arrive. Recently a friend who lives in Paris was making that same trip and planning to do the usual RER B to Gare du Nord and walk to Magenta to take the RER E to Hausman-St Lazare but something was wrong with Line E so she took the RER B to Chatelet-Les-Halles and transferred to Line A and got off at Auber and walked to the station.

Not renting at CDG after landing is a wise choice!
Taxi to Saint Lazare is easy, and the station at Saint Lazare is quite manageable. There's just one big concourse with all the trains, and huge indicators to tell you where to go.

Note that the rental companies in Caen take nice long lunches -- so be aware you may have to wait until the place opens again and go find a nice long lunch somewhere yourself.

I did this exact in December. There are a lot of rental car options across from the train station. It is easy to do.