This question is asking for two very different things. One question asks to find an internal monologue from the book Speak. This should not be too difficult. The story is told from a first-person perspective. Melinda is our narrator, and we get a great look into her thoughts to see the mental and emotional turmoil that she goes through over the course of this one particular school year. Melinda is an emotionally destroyed person as a result of her having been raped the summer before. The book is loaded with all kinds of great monologues on her part about various classes, teachers, classmates, etc. Her commentary on gym class is especially good in my opinion.
The other question being asked here deals with writing a monologue. I suspect that this is the intended question that a teacher is asking. After reading the book, a reader should have a good handle on Melinda's mental state and particular outlook on life. She is depressed, but that doesn't diminish her dark wit. She is quite funny from time to time, and she describes people and situations with a mixture of humor and extreme bluntness. Your monologue should be written as if Melinda is doing the monologue, and you should try to emulate her style. Pick a part from the book that she doesn't necessarily provide readers much detail about, and write a monologue about that. I would focus on doing a monologue about her parents or teachers. There is enough detail in the story about those characters to give you enough information to develop an additional monologue sequence.