How did Johnny's message affect Ponyboy in The Outsiders?

When he is dying in the hospital, Johnny tells Ponyboy to “stay gold.”  He is reminding him about a conversation they had about a Robert Frost poem, “Nothing Gold Can Stay.”  The poem is about how things start out young and fresh, but wither in time.  Metaphorically, it means that people cannot maintain their innocence as they age.  Johnny feels that Ponyboy is still innocent, and he wants him to remain so.  He does not want him to follow the other greasers into gang life.

Pony doesn’t understand that Johnny means at first.  Johnny left a copy of Gone with the Wind for Pony, because they were reading it while hiding out.  It takes him awhile to finally take a look at it, because he doesn’t want to acknowledge that Johnny is actually dead.  In the note, Johnny says that he accepts that he is dying, and it was worth it to save those kids in the fire.

When you're a kid everything's new, dawn. It's just when you get used to everything that it's day. Like the way you dig sunsets, Pony. That's gold. Keep that way, it's a good way to be. … And don't be so bugged over being a greaser. You still have a lot of time to make yourself be what you want. There's still lots of good in the world. (Ch. 12) 

Pony feels sad all over again when he reads the note, because it tells him to tell Dally to look at a sunset, so he can see that there is good in the world.  Dally is dead, robbing a store and then running at the police in his grief over Johnny’s death.  Johnny would not have wanted that, Pony knows. 

Pony decides that he, Johnny, and Dally are not the only ones in this situation. 

It was too vast a problem to be just a personal thing. There should be some help, someone should tell them before it was too late. Someone should tell their side of the story, and maybe people would understand then and wouldn't be so quick to judge a boy by the amount of hair oil he wore. (Ch. 12) 

Pony calls his English teacher to ask how long his paper can be.  Then he writes the story of what happened to Johnny. It is more than just a homework assignment.  It is Pony’s acknowledgement that he will make something of himself, and honor Johnny’s memory.