According to Genesis 3, it was Eve who sinned first. It was she who first ate the fruit from the tree that God had forbidden them to eat from. In Genesis 3:2, we clearly see that Eve knew that God had told her not to eat the fruit from the tree of the knowledge of good and evil. We can assume that she, no less than Adam, knew that she needed to obey God. But then she let the serpent tempt her, causing her to eat the fruit. She intentionally did something that God had told her not to do. She then compounded it by inducing Adam to eat the fruit as well. Since Eve disobeyed God first, she was the one who sinned first.