Tokyo Skytree is a mere four minute walk from Oshiage Station. You will easily see the tower as you leave the station.
From Shinjuku Station to Oshiage Station:
Take the Chuo-Sobu Line (Local) to Kinshicho Station, change trains to the Hanzomon Line to Oshiage Station (44 minutes, ¥390)From Tokyo Station to Oshiage Station:
Take the Marunouchi Line to Otemachi Station, change trains to the Hanzomon Line to Oshiage Station (29 minutes, ¥200)