How elements were created

At the birth of the universe, not all elements existed.
During the big bang, large amounts of Helium and Hydrogen were created, along with some Deuterium and Lithium. High temperatures allowed nuclear fusion to take place, creating some light elements.

Stars came next. Less massive ones could only convert hydrogen to helium, but stars bigger than our sun could convert the helium into elements such as oxygen and carbon. Even bigger ones could make everything up to iron. Nothing higher than iron could be made with fusion.

There is just one more factor left: supernovae. When a star explodes, instead of fusion, neutrons from different atoms collide to make heavier elements, giving us everything else.