In Genesis 3 Satan tempts Adam and Eve in the garden and Adam and Eve sin. When they sin, it breaks their fellowship with God because God can't have fellowship with sin. As a result, sin now passes on to every person who's ever lived. Romans 5:12 says, just as through one man Adam sin entered into the world and death through sin, so death spread to all men because all have sinned. Every single one of us are born sinners. We don't become sinners the first time we disobey God. We're born that way. That's why you never have to teach your children how to misbehave. It comes naturally.
You might be thinking, “then what's the big deal?” If we're all sinners then we are all in the same boat so what’s the problem? The problem is that there is a penalty that comes with sin. Romans 6:23 says the wages of sin is death. This is more than physical death. Sin also brings spiritual death meaning that we are separated from having fellowship with God. Worse yet, ultimately, sin brings eternal death in which we are separated from God forever. Sin is a serious problem and it is because of our sin that God sent His Son, Jesus, to die in our place to pay the penalty for our sin.