Water is the most important nutrient. Each cell in your body needs it. You would die a lot sooner from lack of water than from lack of food. It cools your body when you sweat. If you eat a well balanced diet with a wide variety of foods, you don't need to worry about protein.
Protein can come from both animals and plants.
Animal proteins are more complete and can be more easily used by the body. Vegetable proteins are not as complete. You can make vegetable protein more complete by eating more than one kind at the same time. For example, eating corn and beans. Simple carbohydrates, or sugars, give the body energy, but little else in nutrients.