The Earth is about 4.6 billion years old. During its first billion years there was intense volcanic activity. Volcanoes released large amounts of gas from inside the Earth, and these gases formed the early atmosphere.
The gases from volcanoes were mostly carbon dioxide and water vapour, with smaller amounts of nitrogen, methane and ammonia. The early atmosphere had a large amount of carbon dioxide and little or no oxygen.
This is very different from the atmosphere today, which is about four fifths nitrogen and one fifth oxygen, with only a tiny amount of carbon dioxide. Living things, mainly plants and algae, have changed the atmosphere over time.