The first person to disobey God in the Bible was
Eve, when she ate the fruit from the Tree of the Knowledge of Good and Evil in the Garden of Eden. However, she didn't die immediately.
The first person to disobey God and then die was Adam. While Eve ate the fruit first, God had specifically commanded Adam not to eat it (Genesis 2:16-17). After Eve ate, she gave some to Adam, who also ate (Genesis 3:6). Because Adam was the one to whom the command was directly given, his disobedience is typically considered the initial transgression that brought about death. God said that when they ate of the tree, they would surely die. While they didn't die physically that day, they did die spiritually, and physical death entered the world as a consequence of their sin. So, while not an immediate physical death, Adam's transgression ultimately led to his mortality and eventual death.