A main point, or theme, of the book is the question of personal responsibility, which is something Bryon begins to realize and Mark does not. Stemming directly from this concept is the idea of breaking the cycle of fighting and revenge espoused by the gang mentality through a spirit of forgiveness.
The story also emphasizes the consequences of both growing up and not growing up. Both choices are realistically full of difficulty. Bryon, who tries to act according to a maturing awareness of right and wrong, is left confused and longing for a time when life and decisions were easier. Mark, who does not grow up also longs for that earlier time, and ends in ruin.
I assume that the "main points" refer to the themes of the book. If you go to the links below, you will get complete details of the themes and characters. I will summarize them briefly for you.
This is the story of Bryon Douglas and his journey to manhood when he realizes he must find some sense of his moral responsibility to himself and other people. It is a difficult journey within the environment of gangs, violence, and the use of alcohol and drugs. Bryon realizes he must clean up his act, requiring him to turn Mark into the police in the end.
Mark is like a brother to Bryon, but Bryon can no longer condone the gang life he participated in as a boy. Mark wants to recapture the closeness he and Bryon shared during those years, but Bryon has recognized what gang life has done to himself and to Mark. Bryon tries to get Mark to realize the benefit "when you know your own personality so you don't need the one the gang makes for you." Mark has no conscience, however, and wants to go back to the time "when we were like brothers, not just you and me, but all of us together. We woulda died for each other then."