Speak's structure is heavily based on internal monologue. Because Melinda feels like no one will listen to her, most her emotions are expressed to us through her thoughts. Some of the book's most powerful moments are achieved by allowing the reader to feel like they are "in Melinda's head" through her monologues.
I'm assuming this is for a class project. While I cannot pick an exact passage for you, I would suggest you look at one of the following sections:
- Fizz ed
- The opposite of inspiration is... expiration?
- Rent round 3
- Riding shotgun
- Communication 101
Each of these sections highlights a different aspect of Melinda's downward spiral into depression. The reader is invited to share her inner most thoughts and gain insight into what she's going through.