Adam and Eve were happily engaged in their life in Paradise when a snake came to Eve.
Now the serpent was more cunning than any beast of the field which the Lord God had made. And he said to the woman, Has God indeed said, “You shall not eat of every tree of the garden”? And the woman said to the serpent, “We may eat the fruit of the trees of the garden; but of the fruit of the tree which is in the midst of the garden, God has said, ‘You shall not eat it, nor shall you touch it, lest you die.” Then the serpent said to the woman, “You will not surely die. For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.”
So when the woman saw that the tree was good for food, that it was pleasant to the eyes, and a tree desirable to make one wise, she took of its fruit and ate. She also gave to her husband with her, and he ate. (Gen 3: 1-6)
See how cunning the snake is here? He tells no lies but twists the truth to mislead Eve. He tempted Her. He tricked her.
This is where sin and evil enters into the picture. Being made in the image and likeness of God Adam and Eve both had a free will. They could choose to stay faithful to God and do as He commanded or not.
Here we see both of them choosing to turn away from a trusting relationship with God. They ignore His commandments, thereby separating themselves from Him. This is the nature of sin - to separate from God and disobey Him.