Adam and Eve were sinless and perfect when God created them. But they still could choose to do what was wrong (just like you can choose to do what you aren't supposed to). When the devil tempted them to eat the forbidden fruit, they chose to do it. Although people talk about Adam and Eve eating an "apple" in the Garden of Eden, we don't know what the fruit looked like or how it tasted. All we know is that it was the one fruit in that big, beautiful garden that they weren't supposed to eat. Adam and Eve ate the fruit because the devil started them thinking about what it would be like. Then he lied about God's rule to make them think that God was keeping something good from them. Pretty soon they wanted to eat the fruit more than they wanted to obey God's rule.
The serpent was the craftiest of all the creatures the Lord God had made. So the serpent came to the woman. "Really?" he asked. "None of the fruit in the garden? God says you mustn't eat any of it?" "Of course we may eat it," the woman told him. "It's only the fruit from the tree at the center of the garden that we must not eat. God says we must not eat it or even touch it. If we do, we will die." "That's a lie" the serpent hissed. "You'll not die! God knows very well that as soon as you eat it you will become like him. Your eyes will be opened. You will be able to know good from evil!" The woman was convinced. How lovely and fresh-looking it was! And it would make her so wise! So she ate some of the fruit and gave some to her husband. He ate it too. (Genesis 3:1-6)
Matthew 6:13; 26:41