The exposition is going to be your introduction. It is going to give background info on main characters. In That Was Then, This Is Now the reader is introduced to Mark and Bryon. We learn that Bryon lives with his mom and that Mark is his adopted foster brother. Mark's parents are dead after having killed each other in a drunken fight. The reader also learns that Mark and Bryon are best friends, enjoy hanging out in Charlie's bar, and hustling other players at pool.
The rising action in the story occurs when Bryon and Mark learn that M&M is looking for them. They find him in trouble and save him from getting beat up by a group of greasers. That event thematically ties in with Mark and Bryon meeting Mike Chambers. Mike is in the hospital after having been beat for an offense that he did not commit. Both of those events focus on violence for the purpose of revenge. Bryon is starting to question the legitimacy of that attitude, but Mark still thinks it's good. A wedge is being driven between the two boys, and it continues to widen. Other rising action contributes to this widening as well, like Angela's hair-cutting incident, for example. But probably the final rising action before the climax is Mark beginning to sell drugs.