The word "justice" is thrown around a lot. People talk about "social justice" or a "just war". It impacts (or should!) our daily lives, whenever we find a lost item, pass a person begging for food, or do business. There is much worth sharing from Church teaching on the moral virtue of justice. We'll explore some of these applications later. For now, let's focus on some fundamentals of what "justice" means for an individual.
The Catechism of the Catholic Church tells us that justice "consists in the constant and firm will to give their due to God and neighbor." 
There are two parts to that definition, not because they are different in kind but because they are different in priority. Even a toddler knows that. My son always puts "Hi Daddy, how you doin'?" Before "May I have some milk?" Get straight with your Father first, then worry about everything else.
Before you start to think I have a god complex, let's move on to the point! What do we owe our Father in Heaven? Everything!
Okay, that's easy to say, but do we really understand what we mean and why? God made us from nothing. Everything we have, feel, or do that is good is not ours by right. We were given existence, and there is nothing we could have done to earn it. (It's hard to earn anything when you don't exist. It only takes skipping a few classes in college to learn that lesson!)
How do you give God the everything He is owed, then? Not being infinite minds, we need something more narrow and concrete than just "give everything". God gave us the natural law written on our hearts, what we commonly call our consciences. God made His wants even more obvious, though, it dictating the Ten Commandments. These are clear instruction on what God wants that we can see and hear.
What can we do with this today? Pray for a stronger sense of justice toward God. Meditate on the first three commandments (Exodus 20) and on God's infinitely great gift of existence (Genesis 1). Keep these things in mind through your day. Lastly, make reparation if you have not been just toward God. Find a local confession time and fix it. If you have been unjust to other people, make it up. The Catechism lays out the "how" in paragraph 2487.
Hardon, Fr. John A. The Virtues. Available from http://www.therealpresence.org/archives/Virtues/Virtues_001.htm. Internet; accessed 22 Aug 2010.