The good thing is that AA uses T2, so you will be able to walk to the TGV station without needing to take any kind of shuttle. The walk is all indoor and is aided by moving walkways.

When you get to the Sheraton Hotel lobby entrance, look down and the TGV and RER stations are down below - just take the escalators or elevator. There's a SNCF ticket office there, but as suggested above, try to have your tickets and seat reservations in hand before arriving.