what is the main idea in chapter 6

I think that defining relationships is the main theme here. Cherry Valance helps the Greasers, and thereby creates a bridge of non-violence between the two rival gangs. Ponyboy recognizes that "it wasn't Cherry the Soc who was helping us, it was Cherry the dreamer who watched sunsets and couldn't stand fights." Ponyboy begins to understand that Cherry and he have something in common outside their respective social statuses.

Our understanding of Johnny's character is deepened by his conversation with Dally, in which he asks, trying not to appear eager, "I don't guess my parents are worried about me or anything?" Dally has to answer that his parents haven't asked about him. That his parents don't care about him even when he is wanted for murder and has disappeared explains Johnny's outward meekness -- but it also hints at his inner strength and independence. He is cut off.

The relationship between Dally and Johnny grows stronger in this chapter, and it becomes clear that, while Johnny feels hero-worship toward Dally, Dally wants to protect Johnny and keep him from turning out the way he himself has. As they drive back to the church, he explains, "You get hardened in jail. I don't want that to happen to you. Like it happened to me..." His emotional outburst makes Ponyboy think of him before he was hardened and tough, and reflect on how he came to be that way.