Lots of people in this novel show courage, though it may look differently for each one. Let's take a look:
Boo for attempting a friendship with Jem and Scout despite his reclusiveness; for saving Jem's life without worrying about his own or the consequences.
Miss Maudie for calling it like she sees it--in front of the kids, in front of Miss Stephanie Crawford, and certainly in front of the "foot washin'" Baptists.
Judge Taylor for assigning this case to Atticus because he understood Tom was guilty and Atticus would actually defend him; for keeping order in a courtroom which was so racially charged.
Tom Robinson for hekping a white woman in a world where that was not a prudent--or safe--thing to do and for trying valiantly to escape from prison, though he didn't make it.
Mrs. Henry Lafayette DuBose for kicking her morphine addiction before she died when she certainly didn't have to do so.
Mr. Dolphus Raymond for living his own kind of life despite public scorn and for giving society a lie (that he's a drunkard) to make them feel better about his presence among them.
Mr. Link Deas for speaking up in support of Tom's wife in open court and for acting as her protector when Bob Ewell began tormenting her.
Sheriff Tate for trying to protect Tom from the prejudices of the day, which might have gotten Tom killed without someone like Heck Tate watching.
Mr. Underwood for speaking his mind in a public way (his newspaper) and for helping protect Tom from any harm before the trial.
Dill for trekking alone to the Finches' house (though we don't know the full details because he's such a prolific storyteller).
Aunt Alexandra for coming to help her brother even though she didn't support what he was doing and then for being willing to change her mind.
Jem for sneaking out after dark to retrieve his torn overalls and for confronting the "clan" which had gathered at the jailhouse where Tom was being held.
Scout for speaking frankly to her teacher on the first day of class, for speaking up to her cowardly cousin, and for speaking fearlessly to Mr. Cunningham outside the jail. (In all fairness, in neither case did Scout really understand the potential consequences of her actions, but still.)
Calpurnia for teaching her children to read at a time and place when, clearly, that was not the acceptable or usual practice; for speaking her mind and being willing to punish Scout for her bad behavior; and for tolerating Aunt Alexandra when she comes to stay.
And finally Atticus, for really defending Tom in the face of such prejudice; for facing a rabid dog in a showdown, of sorts; for teaching his children to look beyond skin color (to walk in their moccasins); for recognizing his need for help and inviting his less-than-supportive sister to stay with them; for standing up to Aunt Alexandra; for sitting outside the jail, unarmed, one night to protect an innocent man; for telling Helen in person what happened to Tom; for taking the garbage (and spit) which Bob Ewell dished out; and so many more. For me, he was most heroic when he dared to remind a jury that all men are created equal, knowing full well those twelve men did not believe it.
Some of these are major courageous acts or incidents, while some are much less heroic or even comical. "Courage" is broad enough to encompass them all.
The most obvious examples are:
Atticus's insistence on properly representing Tom Robinson and really trying to win the case, not merely going along with it for show, despite the opposition from Bob Ewell and the confrontation with the Cunninghams (outside the jail the night before the trial). Though Atticus has more allies than you might think.
Boo Radley's leaving the comfort of the confines of his house to save Jem and Scout on "their longest journey together".
More superficially, Jem and Scout overcoming their fear of Boo Radley and becoming more curious than scared of him.
Scout standing up to the Cunninghams as they surround her father and try to get him to let them have Tom Robinson (we assume they want to lynch him).
The Judge (I forget his name) who appointed Atticus in the first place did so for a reason: *he* wanted Tom Robinson to get the best chance at victory in the trial. The Judge could have appointed someone who would just go through the motions and get the expected "guilty" verdict; by appointing Atticus he was making a stand against the status quo (that a black man would ALWAYS be guilty against a white man) and trying, in a small way, to change things. That's quite courageous.
Scout inviting Walter Cunningham to dinner is, in a very small way, a show of courage for both of them, as they both try to understand people quite different to themselves.
Ultimately you could say it's about having the courage to stand up for what you believe in, and having the courage to overcome your prejudices of other.
I hope that's a start for you.