It depends if you are arriving without luggage and want to see the Eiffel Tower as a monument or if you are arriving with luggage and heading to a hotel near the Eiffel Tower.
If you just want to see the Tower, the easiest route is:
Walk to the back of the Gare de Nord station to the métro station called La Chapelle on line 2. Take the métro in the direction of Porte Dauphine to Charles de Gaulle Etoile. There you transfer with métro line 6 to Trocadéro. You'll be arriving across the river from the Eiffel Tower from where you have the best view. Then you can walk to it through the Trocadéro gardens.
If you are arriving with luggage and checking in at a hotel near the Eiffel Tower, take bus 42 which goes from Gare du Nord to the Eiffel Tower. There are several stops near the Tower, from Tour Eiffel to the east to Champs de Mars to the west and the 2 stops in between those two. Depends where you want to go really.
[In normal conditions you would take RER B (direction Robinson/St-Rémy-les-Chevreuse) to St-Michel Notre Dame where you transfer onto RER C (direction Versaille/St-Quentin en Yveline or Pontoise) to Champs de Mars-Tour Eiffel. BUT: there is work on lines B & C this summer so traffic is interrupted on parts of the lines and a shuttle bus takes over]
Here is a map of the subway: http://www.ratp.info/orienter/cv/cv_en/carteparis.php
And a map of the bus routes:
http://www.ratp.info/orienter/cv/cartebus.php?partenaire=
If you tell me your exact situation, the date you will be traveling and what your final destination address is I could help you figure out what the best way is.
And by the way it is very difficult to get lost in Paris, it is not that big and there are enough famous landmarks to find your way with a map or a little help from the locals.