Start a fire. This may seem like a trivial thing on a deserted island, but fire has a number of uses. At the very least, it is a morale booster. You have accomplished your first task, which is key for morale. Fire can also be used to distill water (more on that later), cook, and provide light for you and your rescuers. If you can't start a fire, don't worry, just move on to the next task and keep trying.