Well, if we're going to imagine this island is a tropical one, like the Amazon Rainforest style... we'll need clean water and food. If we're assuming animals(like squrriels and such) are on this island then we can fashion a long stick into a spear. Clean water would usually be obtained from a stream and would need to be filtered in order to keep any diseases out of the water. This can be done with stones or plants. Now shelter can be tricky. We'll need sticks and something like that. We can also use a cave entrance. If there are sheep on the island, kill the sheep and skin the sheep to get wool for a blanket to stay warm. If it's incredibly hot there, cover yourself in mud! It keeps insects away and it'll keep you cool, which is essential in tropical settings.