Take line 4 to Barbes-Rochechouart stationthen take line 2 to Charles-de-Gaulle-Etoile stationthen you're at the top of Champs-Elysées just near Arc de Triomphe
well it is not such a smart idea to walk to "champs-elysees" not shanzelize from the Eiffel tower where there is a perfectly safe and fast underground metro. if you really insist in walking, cross the bridge and walk towards your right until you get to the roundabout at the bottom of trocadero. then you walk up the hill and climb the few flights of stairs. once you get to the top you should be able to the Eiffel tower directly behind you. you will be at a roundabout and you find the road before "avenue kleber" when you walk around anti-clockwise. when you are on that road you should see the Arc De Triomphe almost 500-700 metres ahead of you. keep walking and once you get to the arc, you will be on the champs elysees.