Everything is made up of atoms whether it is natural material or man made material like plastics, synthetic etc. it is the smallest particle in each and every substance. Many atoms combine together to form molecule. A single drop of water has billions of molecules and each molecule of water has two atoms of hydrogen and one atom of oxygen which are attached to each other with the covalent bonding and its chemical formula is H2O. Size of atom in a water droplet is approximately equal to the size of tennis ball on the earth.
Water is made up of a polar molecule which is called as dipole in the scientific language. It is known as polar molecule because hydrogen has less electro negativity than oxygen or oxygen has higher electro negativity than hydrogen. So the charge difference is known as dipole. The molecules of dipole attract each other and finally sticks. That is why hydrogen and oxygen attracts and stick with each other to form water. It sometimes engages in hydrogen bonding.
This is what water is basically made up of but it contains so many things depending upon the source of water. Like salt water have salts and small amount of other substances like iodine, freshwater contains ground pollutants of farming, eroded rocks, drinking water has elements added for purification and rain water consists of atmospheric pollutants and dust particles.