The first thing your survior needs to do when he wakes up on the dinosaur infested island in just his underpants is to quickly find food to prevent him becoming hungry and exhausted and water so he doesn't become thirsty and dehydrated.
To begin with your main source of food will be to collect and eat various berries and other edible plants from the vegetation on the island but as the game progresses and you craft weapons from the resources you find you will be able to hunt creatures for meat which will keep you full and also heal damage. Make sure though that the rank of the creature you decide to hunt is not too high otherwise you will end up being its meal.
To be able to drink water you need to get your survivor into some water, be careful though as not all the water you find on the island is safe to enter.