The chemical composition of water is H₂O, which means that it contains two hydrogen atoms and one oxygen atom. Each one of these atoms has a nucleus and orbitals that surround the nucleus. An onion is a great example of an atom since it has many concentric layers. The very center of the onion represents the nucleus and the outer layers of the onion represent the orbitals.

There is a specific number of electrons that are contained in each orbital, and the total number of electrons is well known for each element that we contain in a group of elements called the periodic table. All elements want to be stable, and they can do this by having a completely filled valence orbital (the valence orbital is the outer most orbital that contains electrons).

Both oxygen and hydrogen do not have completely filled orbitals and that makes them unstable, so they want to do something to make themselves stable.

Hydrogen has only one electron in the valence orbital, but it needs two electrons to be stable. Oxygen has six electrons in the valence orbital, but it needs eight to be stable. By sharing electrons with each other, the two hydrogen atoms and one oxygen atoms can become stable since they will all have completely filled valence orbitals. See picture on this link

When electrons are shared between two atoms, a covalent bond is formed, thus turning oxygen and hydrogen into water.