A metaphor is a direct comparison of two things by saying that object 1 is or was object 2. For instance, "my love is a rose." I'll identify a few metaphors in the novel to help you get started on finding others for yourself.
Although it is not written exactly like the metaphor I gave you as an example, the very first one you'll encounter is the title: Marguerite knows how the caged bird sings because she feels caged in herself.
Another metaphor appears in the Preface: "If growing up is painful for the Southern Black girl, being aware of her displacement is the rust on the razor that threatens the throat."
In chap. 29: "Instead they used their intelligence to pry open the door of rejection and not only became wealthy but got revenge in the bargain."
Now, there are three metaphors for you. You try to find some more.
I believe that "Instead they used their intelligence to pry open the door of rejection and not only became wealthy but got revenge in the bargain." is not a metophor, but an example of personification. Personification is when the author gives inaniamte objects or things human-like traits, and this would be a prime example of personification in I Know Why the Caged Bird Sings.