ESSENTIAL CATHOLIC RESOURCES
Before I start directly addressing any subject, I think it's important to share with you the abundance of resources available about the subject of Catholic Religion, all of which is available online for free.
This is a link to the "New American Bible" which is the English translation of the Bible most commonly used in Liturgies that are celebrated in English.
The Catechism of the Catholic Church
This book summarizes the entirety of the shared Tradition of the Roman Catholic Church throughout the entire world. Probably your most important resource if you are trying to understand specifically what the Catholic Church teaches about something.
The Church Fathers
The Catholic Encyclopedia
Those will most likely be what you will use the most.
But if you are looking for more, here's a directory for the Vatican archive.