By Train
Haneda Airport International Terminal > Shinagawa by Keihin Kyukou line
Shinagawa > Shibuya by JR Yamanote line
By Bus
After customs, go into the arrival hall and check for a stall with orange panel "Limousine bus"
Book a limousine bus to Shibuya in the link below.
Airport Limousine -> http://www.limousinebus.co.jp/en/
You can find more details in Airport official website.
Haneda Airport -> http://www.haneda-airport.jp/inter/en/
After getting off at Shibuya station, you can find the famous and super crowded Shibuya Crossing!!
...Though you should go through a labyrinth before that as Shibuya station, which is under ever lasting construction.
Enjoy your stay in Tokyo!