The earliest writings about Mary are in the New Testament, which was written in the second half of the first century. After that, Mary is mentioned in several apocryphal texts (texts that are written in the style of the Gospels but are not believed to be divinely inspired). One of the most Mary-centric of these books is the Protoevangelium of James, which was written around 150 A.D. It tells the story of Mary’s conception, her childhood, and the birth of Christ, and clearly demonstrates a great interest in Mary’s “backstory,” so to speak.
Mary also shows up in the writing of many early Church fathers, including St. Justin Martyr (who died in 165 A.D.) and St. Irenaeus (who died around 202 A.D.).