Train from CDG to Rouen

The train station is Gare Saint Lazare. Either take a taxi (53€) or if you are travelling light and cash is tight, you can take the bus ("Roissybus") from the airport for about €12/each - it arrives about 600 metres from the station. The suburban train is inconvenient, in my opinion (RER B to Gare du Nord, followed by RER E to St Lazare).
You can book on SNCF Connect (the website of the French railways), or through Trainline for a small fee if you struggle.
Book a departure 4 hours after you land, going into Paris can take a while if there is traffic.

While only some trains (the fastest "train Nomad") accept seat reservations, it is still worth buying the ticket in advance because you can pay as little as €10 if you do so (vs. €24).
This is a fairly recent development, and it is true that you do not have to. If you'd rather just hop on the next train once you get to Gare Saint Lazare, you can do so by just buying a ticket at a machine in the station.

thanks for these responses - generically helpful regarding transferring to rail after landing in CDG.

I'll second the recommendation for the Roissybus. It leaves right outside of arrivals at CDG on a very regular basis, and lets you see Paris as you enter the city rather than enduring tons of steps and changes by tunneling underground. I go to Normandy on a regular basis (sometimes spending a night in the vicinity of Gare Saint Lazare first), and the only time I took a taxi instead, was when I was carrying an inordinate amount of heavy luggage by necessity.