fall of Adam and Eve
The first prophecy as to the coming Messiah occurred right after The Fall of Adam and Eve. God was explaining what the new conditions would be, as they were about to be expelled from the garden of Eden.
When speaking to the serpent, the Lord said:
And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel. Genesis 3:15
In the Douay-Rheims Bible, it is written slightly differently.
In other versions, sometimes “he” is translated “it” or “they”.
Regardless, a woman and her seed are coming. We find that fulfilled in the New Testament with the virgin Mother Mary giving birth to Jesus. Notice in the scripture above it talks about the seed of a woman, not the seed of a man. This refers to the virginity of Mary.