Whether or not if you're a Catholic, or even a Christian, it is well worth your time to follow Fr. Mike Schmitz's series on reading the entire Catechism of the Catholic Church in one year. This is the BEST and most comprehensive way to understand Christianity, and it's history.
I'm posting this on February 26th, and so Fr. Mike is well along in this series, but thanks to the magic of YouTube, you can easily go back and start on Day One no matter the actual date.
The Catechism went through a major overhaul after being basically ignored for 500 years. During the papacy of John Paul II, it was decided that a new, modern catechism was needed, and told Cardinal Ratzinger (later Pope Benedict XVI) to get on it! So here we have a great book about Catholicism from two of the greatest Catholics minds of the 20th century.
I encourage EVERYONE to subscribe and listen to this podcast, as the catechism serves as a great introduction to the Christian faith in a systematic way. It's also a good history of Christianity.
There's no need for you to buy an actual catechism book, but you can if you want to. However, the The United States Conference of Catholic Bishops have published the entire document on their web site, with free access to all.
You can find the online catechism here: https://www.usccb.org/sites/default/files/flipbooks/catechism/
Highly recommended.