There are a number of quotes that you could refer to in this excellent play to elucidate the themes of the individual and justice. You might like to start by considering how Juror Eight defends his decision not to vote for the defendant being guilty like everybody else does:
Look, this boy's been kicked around all his life. You know, living in a slum, his mother dead since he was nine. That's not a very good head start. He's a tough, angry kid. You know why slum kids get that way? Because we knock 'em on the head once a day, every day. I think maybe we owe him a few words. That's all.
This quote therefore says a lot about the dangers of leaping to a conclusion and not giving a defendant a fair hearing. Juror Eight refuses to leap to a guilty verdict and recognizes that the defendant is a human like them who deserves to have his case debated.
You also might like to consider this quote from the beginning of Act Two, which demonstrates the power of the individual and how important the individual can be. When Juror Nine defends his vote for not-guilty, he refers to Juror Eight and praises his stand:
This gentleman chose to stand alone against us. That's his right. It takes a great deal of courage to stand alone even if you believe in something very strongly. He left the verdict up to us. He gambled for support and I gave it to him.
The power of the individual and standing up for our beliefs and convictions is thus praised and recognized.
You also might like to consider the words of Juror 11 at the beginning of Act III:
We have a responsibility. This is a remarkable thing about democracy. We are... what is the word? ... Ah, notified! That we are notified by mail to come down to this place and decide on the guilt or innocence of a man we have not known before. We have nothing to gain or lose by our verdict. This is one of the reasons why we are strong. We should not make it a personal thing.
This quote of course relates to the justice system and how "strong" it is beacause of the way that it is structured. Of course, at the same time, you could argue that the play shows the innate fragility of justice as this case does very quickly become a personal thing.