There are a few main things that Boo Radley does that end up making him get confined to his house for all those years.
First, he started hanging out with the wrong kinds of kids. They were the closest thing Maycomb had to a gang. They did things like getting drunk.
The stole a car and had a joyride, then they locked the semi-police guy in an outhouse. The rest of the boys were sent to reform school, but Mr. Radley locked Boo in and he was pretty much never seen again. He was noticed again when he stabbed his dad, but that was it.