After you spawn in your right is a special thing, that is a sugarcane standing by the water. Close the sugarcane is a sand blocks, it marked by the direction of the arrow in the images below. To finding the main section of this seed you just start dig from that sand blocks, soon you will realize the sound of the spider close the moss stone, keep digging until you fall down to the dungeon. And you will see a spider spawner with lots of spider and of course have a chest with awesome items near by the spider spawner.
Seeds : mushroom Work in PE ver0.10.0+ or ver0.10.5