Answer:
Johnny decided to turn himself in to the police because he is tired of hiding and making Ponyboy hide as well. He thinks that with Cherry's (a soc girl) testimony on Johnny's behalf stating that he had killed Bob (the soc) in self defence will help him get an easier way out.
Explanation:
The Outsiders novel is about how Ponyboy Curtis, a lower class boy, member of a "gang" named the greasers, has to face the rivalry of the two different social classes of teenagers, The Greasers ( lower class) and The Socs (higher class). Feeling like the society has left them as outsiders.
It all starts when Johnny, a greaser, in self defence and Ponyboy's defence, kills Bob, one of the socs. From then on they go into hiding but after a week or so, when they received news of what was going on, Johnny decided it was best if he turned himself in. He based his decision taking into consideration that Cherry (a soc girl, Bob's ex girlfriend) was willing to testify in favour to Johnny statiting that Bob was drunk and attacked them so Johnny was just defending themselves. He was also very disappointed to know that his parents hadn't even asked about him nor were worried about him. He felt as if no one cared and his only family were the greasers, so in a way he owed it to them, he was tired of hiding and making Ponyboy live these type of life. Evaluating all these facts he thought he could maybe get an easy way out, accepting his culpability.
Although his friends tried to talk him out of it, he was determined to turn himself into the police, but on their way back they saw that the church were they were hiding caught on fire and some kids were trapped in it. They sacrifice themselves saving the kids and became heroes. Unfortunately this event left Johnny severely injured causing him death.
The novel is actually a report Ponyboy writes for school telling their story.