What you have so far is a very broad statement: Racism is bad; justice is good. You can get a really strong essay out of exploring the different facets of racism and justice. Yes, the trial of Tom Robinson is a prime example of how racism, not justice, wins in the end, but look at some different ideas of justice:

You could explore Boo's "survival" at the end of the text after he has delivered his own brand of justice to the evil Bob Ewell.

You could even look at how Boo is thought of as evil at the beginning of the book, but the reader eventually discovers just how good a person he is when he saves Scout and Jem.

The racism vs. justice contrast is certainly basic to the novel. You might consider these, as well: kindness vs. cruelty and integrity vs. hypocrisy. There are many instances all of these in the novel, and they are developed through the various characters. Atticus, Maudie, Heck Tate, and Boo Radley himself all demonstrate kindness. Bob Ewell is cruel in his treatment of Mayella and Helen Robinson; his treatment of Jem and Scout exhibits cruelty in the extreme. The town itself inflicts cruelty upon Boo with its gossip and condemnation. Individual acts of racism are cruel. Atticus and Maudie are strong examples of integrity, whereas Miss Gates and the ladies of the Missionary Circle show hypocrisy at its worst. Many incidents in the novel and specific passages of dialog would support these ideas.

Another contrast of good and evil would be courage vs. cowardice. Which characters act with courage to further good, and which characters are cowards with evil results? Again, there are many examples and incidents to show this contrast. Good luck with your essay.