The Catechism of the Catholic Church can be accessed online here. This book contains the teachings of the Church on almost anything you’ll ever want to know about. I will cite the Catechism often in posts as a reference for more information about the teaching being discussed in a post.
As a note for everyone, I will usually cite the Catechism by its full name (The Catechism of the Catholic Church), but I might also use the abbreviation CCC. If you see something that says, for example, CCC#2112, that means I’m referencing the Catechism of the Catholic Church, paragraph 2112. The numbers following the Catechism are always paragraph numbers, not page numbers.