Question: How would you make an atmosphere on the Moon?

I originally thought that simply obtaining water from the Moon, sending an electrical current through it to get Hydrogen and Oxygen was sufficient enough to have humans breathe on the Moon.

But upon further research, I learned that too much oxygen is actually toxic to your body, and it will kill you. Meaning you need the right balance of other gases in the atmosphere to make it breathable.

So my question is simple: What are the required gases needed in the atmosphere, such that humans can breathe and do normal activities? The atmosphere is composed of $78\%$ Nitrogen, $21\%$ Oxygen, $1\%$ Argon and then the rest is traces of other gas. Such as Helium, Carbon Dioxide, Carbon Monoxide, Methane, Krypton, etc.

Which of these gasses should you keep, and which should you drop when creating a breathable atmosphere on the Moon? I think you'll have to keep Nitrogen, Oxygen and Argon, but I'm not sure how you can obtain Nitrogen and Argon.