Sin was first introduced into the world when Adam and Eve disobeyed God. (Genesis 3:1-7) God set in motion a plan that would remove the sin from man. (Gen 3:14-15). This plan required someone to suffer the consequences of our sin. This would only be someone who was free from the guilt of sin. That someone was Jesus. So from that moment on, we see the unfolding of man's history and relationship with God. Scripture records this history in what we call the Old and New Testament.
The Old Testament teaches us how God brought about the birth of his Son, Jesus. The New Testament teaches us how we can obtain salvation through Jesus.