Bereshit 2:9-10 tells that HaShem made to spring up every tree (in Gan Eden) pleasing to the sight, and tov for food and the Etz HaChayim (Tree of Life) also in the midst of Gan Eden, and the Etz HaDa’as Tov v’Rah (knowledge of good and bad). HaShem tells (2:16) Adam explicitly not to eat from the fruit of the tree of good and bad. The rest is history because Adam did eat of it and therefore sinned.
But what exactly was the sin? To disobey, eat, listen to the snake?