Get the RER A into Paris and get off at Charles de Gaulle Etoile
Change onto metro line 6 and go 3 stops to Trocadero
When you get off there you get the best views of the Tower from above and can then walk down through the gardens and past the fountains to get to the Tower.
You don't need to exit the RER station but just change onto the metro line (they are sign posted by their number(metro) or letter (RER)
Personal preference but I'd prefer walking up through the gardens to the tower from the Ecole Militaire - Metro of the same name for reference. You can also take in the Musée de l'Armée and walk down to the Ecole Militaire. All up to you and what you want to see.