Via the METRO/RER: The Eiffel Tower's nearest metro/RER station is Champ de Mars / Tour Eiffel on the RER C train line (yellow line on maps) . From either Terminal 1 or 2 at CDG take the RER B (blue) train line from towards Paris (direction Antony, Robinson, St-Remy-les-Chevreuse). Upon arriving at station St-Michel Notre Dame switch from RER B to RER C (yellow) in the direction of Versailles-Rive Gauche. Descend from the train at station Champ de Mars - Tour Eiffel.

Other stations close to the Eiffel Tower include Bir-Hakeim on Metro line 6, Ecole Militaire on Metro line 8.

Via the Roissy Bus: Take the Roissy Bus non-stop to the Paris Opera by all the big department stores. From there take a taxi to the Eiffel Tower.

Via the AirFrance Bus: Take the AirFrance Bus to the Arch de Triomphe and take a taxi from there. They stow luggage in the belly of the bus. The one way fare on the AirFrance coach is 12€. If you know you will be returning the same way to CDG you can buy a return ticket for 18€. In addition to buying your ticket at the Air France booth, you can also buy them when you board the coach. At CDG the coaches leave from Gate 34 on the arrivals level of Terminal 1 and gate B1 from terminal 2.
It is about a 10 euro taxi ride from the Arch to the Eiffel Tower. There will most likely be taxis waiting when you arrive as the taxi drivers are aware of the coach schedules. An alternative to a taxi is the 92 city bus, getting off at the Ecole Militaire stop. Just walk clockwise to the next street past the Champs Elysses, Avernue Marceau, and you'll see the bus stop. You can buy tickets from the driver.

If you buy return tickets at the airport for the train to Eiffel Tower, the machine will give you 4 identical tickets, make sure that you only use 2 of these to get into the train system and the SAME 2 tickets to get out at the other end. If you inadvertently use 2 to get in and the other 2 to get out, this will void the tickets for the return journey, and you will have to buy more tickets to get back to the airport.