3rd island.. is that the island with the raptors? there's a cave on a hill and in front of this cave should be a sign. read this (use key) to get more information.

Talk to the people that are NOT in the village. There are two.

Get a stone by hitting a rock. Get a stick by hitting a tree or bush. Open your inventory and combine them into a hammer. Take the hammer in your hand. Right click where you want a building to be. Select the building type. Right click the building site until you have no more materials to build. Go get some more of the stuff you need, and right click again until the building is done.

The basic shelter is made from sticks and leaves, both of which can be had by hitting trees and bushes.

Look for a crate that you cannot open by hitting it. The washed-up crate is (unsurprisingly) near the beach. If you go for a walk around the beach of the island, you will find it.