The Catechism of the Catholic Church
This book on the teachings of the Church is more of a reference book but is critical reading for every Catholic. Buy a copy or look up one of the many available online and browse through the Table of Contents or index. Find a few topics that interest you and start reading. Set a goal of reading a few passages once a week.
If you have older children or teens, check out YouCat, which is a version of the Catechism for youth. It’s also a good introduction for adults who feel overwhelmed by the full version.