99% of the human body is made up of oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. However, I think the answer you are looking for is water. Water makes up about 60% to 70% of the human body. The exact amount depends on the individual, and whether we are referring to a man, woman, or child. Various organs have varying water content in them. For instance, the heart has 73% water, the bones have 31% water and the skin has 64%. The lungs have 83% water. Now you can see that it makes that full grown humans need between 2.2 and 3 liters of water each day while children need proportionately less.
The answer is water. Water comprises about 70% of the human body, thus the body is mostly oxygen and hydrogen atoms bound together as water. Other organic molecules that make up the body in order or decreasing prevalence are proteins (made up of amino acids), lipids (fat molecules), carbohydrates (everything from simply food sugars to complex starches used for structural purposes), nucleic acids (DNA and RNA), inorganic ions (like calcium and magnesium), and dissolved gasses like carbon dioxide and oxygen. Other chemicals like vitamins and tooth enamel make up the rest of the body.