Route 1:
Take RER LIne A from Marne-Le-Vall�e to Gare du Lyon. Walk the short distance to Gare d'Austerlitz, crossing the Seine on the Pont Charles de Gaulle. Take RER Line C (with the destination shown as towards Versaille-Rive Gauche & Saint Quentin-en-Yvelines) to Champ de Mars-Tour Eiffel.
Route 2:
Basically as Route 1 but remain on Line A until Chatelet les Halles. Then walk over the bridge to pick up LIne C at St Michel-Notre Dame. (The advantage of this route is that allows you to see Notre Dame cathedral and/or visit the Latin Quarter where you'll find loads of cheap restaurants).
Route 3:
Basically the same as Route 2 but, if you're feeling lazy you can take Metro Line 4 (for 2 stops towards Porte d'Orl�ans) to get you from Chatelet-Les Halles to St Michel-Notre Dame.
Route 4:
Remain on RER LIne A until Charles de Gaulle Etoile. Take Metro LIne 6 and alight at Trocadero. This is alongside the Palais de Chaillot and is an excellent way to approach the Eiffel Tower (which is directly across the Seine, over the Pont d'Iena). Some of the best photographic opportunities, of the Eiffel Tower, are from the Palais de Chaillot with the fountains in the foreground.
Route 5:
Only take RER Line A as far as Nation. Then take Metro Line 6 to either Bir-Hakeim or Trocadero.
Out of that lot, I'd choose either Route 2 or Route 4. If you break your journey, on Route 4, at Charles de Gaulle Etoile, you'll find that you come out of the station alongside the Arc de Triomphe, at the top of the Champs-Elys�es.
Chris