Typically, the protagonist of a text is either the character speaking (first person narration) or the character the story is about (third person narration). The antagonist of any piece is the character who conflicts with the protagonist. Many times, the protagonist can face multiple antagonists over the course of the text.
In regards to Harper Lee's novel To Kill a Mockingbird, the protagonist of the novel is Scout Finch. Scout sets herself as the protagonist of the novel from the very first line of the novel.
When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow.
Here, Scout uses the pronoun "my" to denote the fact that the story readers are about to read is her own. This is even more solidified as the second paragraph begins. The narrator, unnamed at this point, continues to use personal pronouns (us, I, and my).
Curiously enough, Scout's name is not mentioned until a few pages in (page four on the electronic copy used). Prior to this, Scout details her family's history and how they cam to live in Maycomb. The first time readers come to know the name of the protagonist, and narrator, is when "Dill came to us."
Jem, Scout's brother, is speaking to Dill when he first uses Scout's name.
“I’m Charles Baker Harris,” he said. “I can read.”
“So what?” I said.
“I just thought you’d like to know I can read. You got anything needs readin‘ I can do it…”
“How old are you,” asked Jem, “four-and-a-half?”
“Goin‘ on seven.”
“Shoot no wonder, then,” said Jem, jerking his thumb at me. “Scout yonder’s been readin‘ ever since she was born, and she ain’t even started to school yet.
As readers meet Dill, they are also introduced to Scout--the narrative voice behind the novel.