Best way to get to Gare de Lyon from CDG

Taxi is a flat rate of 50 euros. There's also a direct bus called Le Bus Direct, line 4.

Absolutely taxi. Landing at 11:30 gets you out of CDG at 12:30, if all runs smoothly. That is barely enough time to find the trains and purchase the tickets.

Thanks so much!

Gare de Lyon has a somewhat confusing layout.
It is divided into two halves, "Hall 1" (the old part), and "Hall 2" (the new part). Hall 3 is the underground area.
Hall 1 has platforms with letters, A to N
Hall 2 has platforms with numbers, 5 to 23, odd numbers only.

See plan here: http://www.garedelyon.fr/wp-content/uploads/2013/10/plan-halls-gare-de-lyon.png

When you arrive (by taxi, bus or métro), look at the video screens to find your train. It will list it as either "Hall 1" or "Hall 2". This allows you to go to the correct part of the station.
About 15 minutes before departure, the screens will change to show the actual platform, but since you are already in the correct "Hall", you only have a short distance to walk.
There are a limited number of seats, if you find an empty one, grab it!

Best way may be taxi.

Another option is RER B to Chatelet then RER A to Gare de Lyon. I'd do the RER, but that's me. I pack very light and I feel like surface traffic is always an unknown. I work in DC and traffic here can be dead stop, due to an accident or construction, anytime day or night. I go out of my way to stay off the road around cities, including taxi or bus, whenever possible.

Thanks for the good information on Gare de Lyon. I'm taking a train to Lyon in September and it's my first time out of that station.

or option C, as someone mentioned above, Bus Direct Line 4 for 17 euros. If you're traveling alone (if two people, the price differential between that and a taxi starts to diminish in favor of the taxi). But for a solo traveler, 17 is a lot less than 50, and Bus Direct is WAY comfier than the RER. And if your plane arrives on time and assuming an hour to get through immigration, pick up your bag(s) and find the bus, you're well within the time frame for this to work.

I just took it last week!

Taxi, but only registered taxis. Ones that know which terminal to take you too by where your are flying to and with a set price.

It depends on traffic, but usually a taxi. The bus is also very good, and as fast if the timing is right.