How did the Jewish Festival Purim being?

Okay so im working on a project for school and I know everything else i need for it but i just need to know it began (the festival i mean) like the only close thing i can find is "Purim is a festive Jewish holiday recounting the deliverance of the Jewish people from extermination plot by Haman the Agagite" but that is what Purim is. I need to know how it started . Thank you for your help !