In my opinion, based, of course, on your topic, either of these would make a good strong representative quote for I Know Why the Caged Bird Sings.
"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. It is an unnecessary insult."
"A light shade had been pulled down between the Black community and all things white, but one could see through it enough to develop a fear-admiration-contempt for the white “things”—white folks’ cars and white glistening houses and their children and their women. But above all, their wealth that allowed them to waste was the most enviable."
"My race groaned. It was our people falling. It was another lynching, yet another Black man hanging on a tree. One more woman ambushed and raped. . . . This might be the end of the world. If Joe lost we were back in slavery and beyond help. It would all be true, the accusations that we were lower types of human beings. Only a little higher than the apes."
"The Black female is assaulted in her tender years by all those common forces of nature at the same time that she is caught in the tripartite crossfire of masculine prejudice, white illogical hate and Black lack of power. The fact that the adult American Negro female emerges a formidable character is often met with amazement, distaste and even belligerence."
It's good that you have a thesis developed for your paper, but I'd suggest making it more specific and ambitious. Right now it is more of a factual statement; what will you prove or show about her life?
That said, I'd look at the sorts of things that define Angelou's early life and find one for each one: family (perhaps individual family members), places, race, and perhaps ethical action (doing the right thing).
One of my favorite (terrible) lines is "If you ever tell anybody what we did, I'll have to kill Bailey." Mr. Freeman said this after the first time he abused her.
Another equally scary line is "Every person I knew had a hellish horror of being 'called out of his name'." That one's about whites renaming blacks, and is scary because it is so universal.
In general, though, you can find quotations by looking for and at representative areas.