Adam committed sin by disobeying God's command not to eat from the tree of the knowledge of good and evil.
That disobedience shows that he choosed to rely on his own judgement and desire for wisdom like God, than to submit to God's authority.
God gave Adam a warning that if he eats from the tree of the knowledge of good and evil that he will die.
The serpent tempted eve, promising her that they will become like God if they know the good and the evil.
Eve agreed and ate the fruit then she gave some to Adam and he ate it.
The sin that Adam committed was not just eating the fruit, it means that he rejected God's authority, he has a desire to be self governing and lack of being thankful which leads to having a desire to be like God.
By deciding to eat the forbidden fruit, Adam and Eve choose to decide for themselves what was right and wrong ignoring God's role as the ultimate authority.
They sought to become wise and knowledgeable like God, putting themselves in God's place.
Their act cause them to spiritual separation from God, leading them to physical death and suffering.
It was stated in the book of Romans that sin and death entered the world through Adam which affected all humans.