Christian theology uses Scripture, tradition, experience, reason, and nature to discover truths about God, with the priority being Scripture. Theology is about bringing these elements into an operative framework to inspire worship, doctrinal development, and spiritual growth. It's informational and formational. If you have ever thought about the nature and implications of belief in God, then you have engaged in theology. There are various levels and avenues of theological studies, so I will do more recommendations in this area. For now, here are three books that I would recommend for getting started with theology.
A Little Book for New Theologians — Kelly Kapic — This book is exactly what it sounds like. It is short and a great resource for starting a journey in theological development. It is very reader-friendly and offers a lot of depth for such a small book. Several universities use Kapic’s book for entry-level theology courses, and I have used it for discipleship and internship material. If you’re on a tight budget, but still want to read something of high quality, this is a great little book to get.
Bringing Theology to Life — Darren Marks- I accidentally found this book a couple of years ago, and I am glad I did. Marks is a thoughtful theologian who does a good job of providing engaging surveys of fundamental doctrines for the Christian faith. He provides some helpful (and not dull) history and offers ways on how to make theology relevant for our culture without losing or softening biblical convictions.
Theology for the Community of God — Stanley Grenz — This book is one of my most consulted books for teaching and personal study. It is a big book, but it is not overly technical. Grenz does a great job of making high theology accessible and offers accurate descriptions of various positions within Christianity. This is probably my personal favorite book on systematic theology. As the title implies, Grenz makes theology practical for the Christian community, not just the individual studier. It it is roughly thirty dollars, but the amount of content is worth the price.
Other helpful books. Some additional resources would be The Heart of the Gospel by Robert Coleman, What Christians Ought to Believe by Michael F. Bird, and Christian Doctrine by Millard Erickson.