What a rich novel. There are so many themes you can choose from and support well with textual evidence. Here are some ideas:

People are inherently good—or evil. You'd need to decide where you land on this debate, but you could support either one well with textual evidence. On one hand, you have the Ewells and the town in general, who send an innocent man to his death based on racial prejudices. On the other hand, you have Atticus, Miss Maudie, and Boo Radley, who help people even when it's not expected or even frowned upon. I tend to think that Lee makes a stronger case for the innate goodness in humanity, but I'm a bit of an optimist that way. With Bob Ewell's death at the end, you could make an argument that good (represented by Boo here) will always win over evil in the end.

Growing into an adult requires a loss of childlike innocence. Before the trial, Scout and Jem spend most of their days lost in innocent games with Dill and concocting mysterious stories about Boo Radley. And suddenly, the trial changes everything. Since Jem is older, this seems to affect him most, and he really struggles with how the town could have convicted Tom when Atticus clearly proved his innocence. Thankfully, Atticus provides a great support in helping Jem interpret what has happened.

Careful and thoughtful parenting can overcome lots of life's hurt. Atticus isn't the most traditional parent. After all, his children call him by his first name. But when it matters, Atticus is always there for his children. When Scout can't understand why her teacher forbids her to read, Atticus smooths things over. And when the children want to know about the trial, Atticus explains things to them in ways they can understand and process. He is warm and attentive, and he really provides them with the foundation they need to avoid melding into the racist attitudes of their small town.

Sometimes bad things happen to good people. The most prominent example of this, of course, is when Tom is killed after he's imprisoned unjustly. But we also see that Atticus is met with anger and hostility for taking Tom's case; this is particularly true after the trial when Bob Ewell threatens to kill him. And the kids are almost murdered by Bob Ewell at the end of the novel; Jem suffers a broken bone in the attack. Sometimes people are just cruel—but there is still hope that goodness will persevere.

There are a number of themes explored by Harper Lee in her novel, To Kill a Mockingbird. Among them:

LOSS OF INNOCENCE.  This theme primarily involves the three children--Jem, Scout and Dill--who are exposed to a number of life-altering experiences over the course of the novel.

RACIAL PREJUDICE.  Although the Tom Robinson trial is the prime example, there are other obvious examples of the intolerant attitudes by white Maycomb citizens toward their African-American neighbors. The church group, who pretends to offer aid to the Mruna tribe in Africa while scorning their own black neighbors, is just one such case.

COURAGE VS. COWARDICE.  The children show great courage in standing up for their beliefs under duress, while Boo Radley shows his own bravery when he comes to the rescue of the children. Bob Ewell is the most obvious example of the coward, a man who preys on children in order to seek revenge against Atticus. The jurors, who refuse to accept the evidence before them at the trial, are another example.

KNOWLEDGE VS. IGNORANCE.  The author seems to group Scout's various teachers among the ignorant members of the town. Miss Caroline tries to forbid Scout from reading with her father, and Miss Gates spouts her defense of the Jews in Germany while spewing her own bigotry against Maycomb's African-American community. Dolphus Raymond explains to Dill that he is too young to understand all the hatred around him, but that he will understand it better when he becomes older.