I would say Pony is a hero in two ways. He is a hero in the conventional sense for helping to save the kids and he is a hero in a less conventional sense for overcoming his problems.
We usually think of heroes as people who rescue other people or do something equally brave. Pony did this, and so he is a hero in that way.
But to me, a hero is also just someone we can or should admire. And I think we should admire Pony for overcoming all the problems he has and the temptations to become a bad person. I think Pony is admirable because he is going to "stay gold."
I think the other answers to this question are great, and I definitely agree with them. I also think Ponyboy Curtis can be considered a hero in S.E. Hinton's The Outsiders, because he is a survivor. Losing both parents at a young age is incredibly hard on a child, but Ponyboy and his brothers are able to make it on their own, even if it's by just barely getting by every month. I also have a tremendous amount of respect for Ponyboy because he knows the value of education. He is a dedicated student and a hard worker. Most kids would have just given up, thinking their futures futile if they had no money and no parents. Ponyboy, however, continues to work hard and do well, with the hope of receiving a college scholarship. He could have very easily gone the other way and decided school wasn't for him. Instead, though, he makes the wiser decision to stay in school and do well.
Initially, Pony Boy was courageous when he ran away with Johnny. He could have stayed behind and placed the blame on Johnny for killing the Soc, but he did not. Instead, he risked his own safety and possible consequences by running with Johnny. He knew his friend had protected him so he was not going to put a blame on his friend.
Pony Boy is also a hero because he ran into the church to save the children. He was not going to allow Johnny to try and save them alone. After Johnny dies and Dally has gone crazy and has the gun, even though he knows it is unloaded, Pony Boy puts himself out trying to prevent the police from shooting Dally.
The last way in which he was a hero is in his willingness to take heed to Johnny's words and to step forward to make changes in his life. He demonstrates this by writing the story about their lives.