The first task to accomplish is to locate the four raft materials Kairi has asked you to find. You can find the first of two Logs on the shore underneath the bridge, not too far from where you spoke with Kairi.
Travel across the beach and up to the dock where Selphie is sitting. Go up the stairs and take a right up another set of stairs. Head around the path at the base of large tree until you find a ladder. Climb the ladder, and head along the wooden platform and into a small tree house. Inside, on the right wall, is the Cloth.
Head all the way back down toward the dock, and climb the ladder opposite the stairs that led up to the tree house. This ladder will lead you up to where Tidus is, and just a little ways behind him is the Rope.
Finally, head back down to the beach and enter the Seaside Shack. Off to the right is a Save Point, which you can use if you want. Go up the stairs and out onto the bridge that leads out to the small island. There, you’ll find Riku sitting on the tree, and toward the back of the small island, you’ll locate the second Log. With everything collected, you can return to Kairi, but it’s a good idea to explore the island some more and maybe even challenge some of your friends.