The straightforward answer to the question is this: sin entered the world through one man, and death through sin. (Romans 5:12)
The first man and woman (Adam and Eve) were given great freedom, but also one serious warning. You must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die. (Genesis 2:17) This was an ideal test of man's willingness to obey what God said simply because God said it. But the devil tempted Eve to disbelieve and disobey God's words, and she did. When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it. (Genesis 3:6)
Read more: What went wrong?