This is a great question. In a sense, the whole work is about justice. From this point of view, I will mention a few of the most important places where Plato speaks of justice.
In books 2-4, Plato speaks of a political body that is broken up into three parts. According to Plato an ideal society consists of three main groups of people. There are producers, warriors and guardians (or rulers). As you can imagine, producers such as craftsmen and farmers produce things, warriors guard the city, and guardians rules through use of philosophy. Plato argues that when society is structured in such a way, justice is the outcome. In other words, when people do what they are created to do and not interfere, then there is justice.
In book 4 (at the end), Plato begins to talk about individual justice. Like the city, the individual has three parts. A person has the rational part, the spirited part, and the appetitive part. For individual justice to exist, the rational part has to subdue both parts, especially the appetitive part. When this happens, there is personal justice.
Finally, in book 9, Plato argues that justice ought to be sought for the sake of justice itself. More importantly, it is the philosopher king that would chart this course.