Without knowing any context, I would recommend getting a textbook. I have The Musician's Guide to Theory and Analysis (by Clendinning, Marvin). The book is well

Without knowing any context, I would recommend getting a textbook. I have The Musician's Guide to Theory and Analysis (by Clendinning, Marvin). The book is well-structured, reasonably clear, and comprehensive. The main risks when you go self-taught are that you have gaps in your knowledge or that you learn things in an order which makes it harder to learn. Textbooks are great—they give you an order to learn things in, where each piece builds on previous pieces. That said, it’s hard to give more specific advice without understanding your goals more. Are you trying to understand songs? Are you trying to write songs? Are you interested in specific genres? I will give a few other tips: - Don’t think too hard about the rules. - Don’t get too distracted by modes and scales. Major and minor will carry you far. - Many books cover counterpoint. You can skip it. Think of counterpoint as a learning exercise that gives you a set of techniques for dealing with multiple voices when composing music. The rules are there to teach you those techniques.