The above answers are not quite correct. Boo got in trouble because he got mixed up with the wrong crowd. He did not do anything horrible; he was just being a wild youth. Most importantly, he did not go to jail, even for a little. Here is what the text says:
They did little, but enough to be discussed by the town and publicly warned from three pulpits: they hung around the barbershop; they rode the bus to Abbottsville on Sundays and went to the picture show; they attended dances at the county’s riverside gambling hell, the Dew-Drop Inn & Fishing Camp; they experimented with stumphole whiskey. Nobody in Maycomb had nerve enough to tell Mr. Radley that his boy was in with the wrong crowd.
One night, in an excessive spurt of high spirits, the boys backed around the square in a borrowed flivver, resisted arrest by Maycomb’s ancient beadle, Mr. Conner, and locked him in the courthouse outhouse.
The whole bit about Boo being in county jail was a rumor that Miss Stephanie perpetuated. She was the town gossip.
Most of the information that Jem and Scout receive about Boo Radley is from Miss Stephanie Crawford, the town gossip. However, from the evidence in chapter one, it says that Boo Radley was hanging out with a bunch of the Cunninghams when they resisted arrest and put Mr. Conner in the Courthouse outhouse, not the other way around. Then, when Mr. Conner brought up charges against the boys who had been driving around backwards in the square, and who put him in the outhouse, the judge sent the boys to a "state industrial school." Mr. Radley thought that was like prison and asked to have Boo released to him with the promise that it would never happen again. That is when Boo's home imprisonment began.
After the scissors incident, Mr. Radley said it might be good to lock Boo up, but not to charge him with anything because he isn't a criminal. Mr. Radley also refused to send Boo to a mental asylum. "The sheriff hadn't the heart to put him in jail alongside Negroes, so Boo was locked in the courthouse basement" (11). The evidence clearly states that Boo did not go to jail, but to the courthouse basement instead. Due to mold in the basement and the cost to house Boo there, the county asked Mr. Radley to take him back after a while. So, no--Boo Radley never went to the county jail. If he had gone to jail it would have been after the scissors incident and not after the joyriding one.
Boo Radley was thrown in jail for joyriding in a Ford Model T, also, Boo Radley was arrested for nearly stabbing his father to death with a pair of scissors, he grew angry with his father and took a pair of scissors and stabbed him in the leg with them, which landed him in the societal position that he was in, there making him the social outcast.