This is not the sort of question to put here, where you will get a one paragraph answer. You need a book or at least a long article to describe the process whereby the Earth spawned life around a billion years after it came into existence itself . Even this would be a theory not an established set of facts. There is evidence that some form of living organism existed as long ago as 31/2 million years past.In fact, Billion year old zircon found in Western Australia contained high amounts of a form of carbon typically used in biological processes.
Increasingly complex living forms developed from molecules that could replicate themselves, just as DNA does. There seem to be two main explanations. Most believe that these organisms developed from gradual changes on Earth. Water percolated down rock into seas of iron sulphur causing a hydrogen reaction and carbon dioxide.
Ultimately the chemical that powers living cells enabled amino acids, which are the building blocks of proteins to be formed. They also formed nucleotids, the building blocks for RNA and DNA. When those molecules became able to reproduce themselves the basis for life-forms existed. The other possibility that receives serious consideration is that life forms actually came from another planet. Richard Dawkins has created a credible argument for this.
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