Take the South Exit from Ueno Station and cross the main road directly in front of you. On the other side of the road you will see many small streets that lead away from the train station. Follow any of the small streets and after a few minutes you will arrive at Ameyoko.
From Shinjuku Station to Ueno Station:
Take the Yamanote Line direct to Ueno Station (26 minutes, ¥200)
From Tokyo Station to Ueno Station:
Take the Yamanote Line direct to Ueno Station (9 minutes, ¥160)