What was the first element created after the Big Bang?

Hydrogen - and for the first few hundred thousand years there were no atoms at all, just a plasma too hot for electrons to stay attached to nuclei. Protons formed within the first second, but a proton with an electron bound to it, which is what a hydrogen atom is, had to wait about 380,000 years for the universe to cool enough.

That moment is the oldest thing anyone can see. When atoms formed, the universe stopped being opaque and light could travel freely for the first time - and that light is still arriving, stretched by expansion into microwaves. The cosmic microwave background is a photograph of the instant the fog cleared, and its tiny temperature variations are the seeds from which every galaxy grew.

Nucleosynthesis in those first minutes made hydrogen, helium and traces of lithium, and then stopped - the universe expanded and cooled too fast for anything heavier. Everything else, all the carbon and oxygen and iron, was made much later inside stars and in the explosions that ended them. The predicted proportions of hydrogen and helium from those first minutes match what is actually observed with considerable precision, which is one of the strongest pieces of evidence for the whole model.