Taxi. Count on a hour in transit if it's rush hour and about 60 euros.Assuming you're buying tickets ahead of time on the SNCF website, give yourself plenty of time (like 3 hours) between landing (assuming it's an early-morning arrival from the USA, to boarding the train.

Taxi will be relatively cheap and faster than other options from the customs exit to the Gare Austerlitz front door. The fare should be around 50€ to 55€ plus 3€ additional each for the 4th and 5th passengers, plus 1€ per each bag placed into the truck (after the first which is not charged). I typically tip 3% to 5%.

I would also emphasize that you should allow 3 hours for unknown airport delays (mechanicals, weather problems, airport traffic congestion) 1 hour travel time to Gare Austerlitz, and additional time if you need to pick up or purchase tickets.

I have already purchased the tickets. all we need to do is get there! i have an 8 hour layover in paris until train. i was advised to rent a locker at d'austerlitz for our luggage. i also heard that taking a train ride from the CDG was a good option. however, reading these two replies, both suggest a taxi. so before leaving USA it would be advised to come with some euros already in hand?
also, where is good place to exchange us dollars to euros once i arrive paris, france?

"so before leaving USA it would be advised to come with some euros already in hand? also, where is good place to exchange us dollars to euros once i arrive paris, france?

You don't want to 'exchange' $$ for €€ anywhere. And you don't need to bring € w/ you. You simply walk up to one of the cash machines at the airport and use your ATM/debit card (make sure it is linked to your checking account)

That is what you should do any time during your trip.