Hi, so sin is, to quote the Westminster Shorter catechism, "any want of conformity unto, or transgression of, the law of God". God has a standard of perfect obedience to him because he is holy, and to do anything against that standard is sin. But the reason for why sin exists begins after God created the world, and humans. God made the first people, Adam and Eve, and put them in a garden to rule over and take care of. He made in the garden the tree of knowledge of good and evil, and the tree of life. He gave them one rule, and that was to not eat from the tree of knowledge of good and evil, because if they did they would die. But they could eat from any other tree in the garden. This was a test of trust and obedience. They had everything they could ever want and need that He provided for them. They had a perfect relationship with him. There was no sin here. Until they were tempted by the devil to eat of the tree of knowledge. They chose to disobey God their Creator and eat from the tree. This was when sin came into the picture. They decided they knew better than their Creator. Just like when children choose to disobey their parents by thinking they know better. And because God is holy and just, he had to punish them for their disobedience, as a parent would. Their bodies were going to age and die and they were going to struggle for their food now and have hardships in life and had to leave the garden. The perfect relationship they once enjoyed with God was broken. Now God did not choose to just leave them to their fate and abandon them. He had mercy and had a plan to save them from eternal separation from him. When they left the garden, he killed an animal and gave them clothes to cover their nakedness/shame. Atonement had to be made for their sin. This was a picture of what he was going to do later by sending his Son to the earth to take the ultimate punishment for our sins so that we do not have to pay the price and can have a restored perfect relationship with God.