Sin is mentioned hundreds of times in the Bible, beginning in the book of Genesis, with Adam and Eve's "Original Sin." When Adam and Eve ate from the Tree of Knowledge they committed the very first sin, the original sin. Eating from the Tree of Knowledge was sinful, because they gave in to the temptation of the snake and didn't listen to what God asked them. You can think of this situation as being you not listening to your parents telling you to clean your room. You didn't do what your parents asked of you, you sinned...
Sinning breaks the trust in our relationship with God. A trust we need to fix quickly so we can get back on the right road to Heaven. In order to fix our relationship with God, Reconciliation was created. The Sacrament of Reconciliation is our opportunity to show God how sorry we are for our sins, and ask for our forgiveness.
How do we know that we've sinned?
Ultimately, sinning is not following God's instructions. But what are God's instructions? In Exodus 20 God gave us 10 commandments, 10 instructions that us Christians should try to follow in our everyday lives. If we find ourselves breaking those rules or regulations often, this is when we know we have sinned. Here are some resources of the Ten Commandments to guide you: