La D�fense (RER & Metro) station is only just across the Seine from some of the principal attractions of Paris.
The RER suburban trains provide a sort of 'limited stop' underground service. (Always take an RER train, in preference to a Metro service, where there's a suitable line available. They're quicker and tend to get much less crowded. You can use a 'Paris Visite' ticket on RER trains, as well as Metro and buses). La Defense is just one RER stop (about 5 minutes) away from Charles de Gaulle �toile station, which is right next to the Arc de Triomphe and at the top of the Avenue de Champs-Elys�es.
From Charles de Gaulle �toile station, just 3 stops on Metro line 6 (not much more than 5 minutes journey) takes you to Trocadero station. That's by the Palais de Chaillot which is directly across the river bridge from the Eiffel Tower. So you can see that you're close to plenty of attractions.
Visit the RATP site for information about getting around:
http://www.ratp.fr/
(Click on the Union flag if your French isn't too good!)
Also consider using the Batobus river service. You buy an all day ticket and then hop on and off as you please. The Batobus runs from the Eiffel Tower to Notre Dame Cathedral, calling at some of the principal tourist attractions en route. It's a lovely way to see the city (and far cheaper than the over-priced guided river tours):
http://www.batobus.com/english/8escale.htm
Chris