We've already looked at a bit of the Church's teaching on justice toward God. That comes first. That must come first. Without a right relationship with God, we are handicapped in our ability to practice justice toward any of His creatures. (John 15:5)
Remembering that big "if", let's continue and consider justice toward other people.
"Justice is not charity. Justice is a strict obligation." (Hardon) The Catechism quotes St. John Chrysostom on this point: "Not to enable the poor to share in our goods is to steal from them... (t)he goods we possess are not ours...." This is paying the debt of justice, not performing charity. 
Wait. What? So when I give to charity, I'm not doing charity?
Let's think about it this way: When you return a book to the library so the next person can borrow it, are you doing them a favor? No, you're simply following the rules of the librbary. The book isn't yours, but it was loaned to you to use and give back for the next person.
Everything you have comes from God. He gave you the mind and body to earn things, the talent to make them; He created all the material things in the beginning. We only subcreate. ("Let there be iPhones!") Creation was given to you just like that library book, to be used but also to be shared as a necessary part of the arrangement. You can't return the library book on time and expect a parade. You met the requirements of justice and no more.
So how do we do even this minimum? How do we give what is due? Jesus was asked that by a crowd (talk of axes and hellfire has a way of grabbing the attention) and He laid it out. Do you job well. Share what you have with those that don't. (Luke 3:10-14) Pray for an internal change, for help in changing your mindset. Giving these things is a minimum and the debt of justice. Charity comes after.
Hardon, Fr. John A. The Virtues. Available from http://www.therealpresence.org/archives/Virtues/Virtues_001.htm. Internet; accessed 22 Aug 2010