Reddit mentions of Music in Theory and Practice, Volume 1 (v. 1)

By "understand music" I'm assuming you mean like music theory?
www.musictheory.net is a wonderful resource to start you off, some of it may seem too easy at first but stick to it and it'll get challenging. Past that, I would honestly recommend picking up a textbook. Music in Theory and Practice is the best damn textbook you can get that takes you from "what are notes" to late romantic era macro analysis of chord relationships. It's pricey but it's good. Tonal Harmony is another good one, and is a much better deal I linked to one on Amazon for $50. I know spending a lot of money on a textbook seems like a waste when we have the Internet, but they are both textbooks that you will keep and continue to use and reference for years. I own both of those because I needed them for music theory classes at different universities, but even without a professor to lecture, the material in there is pretty self explanatory. Let me know if you have any questions / want a better explanation, I hope I understood your question correctly!

Edit: I forgot to add /r/musictheory is available too, but a lot of posts there assume the reader already knows quite a bit about theory, it isn't exactly a resource for someone wanting to begin learning. The sidebar there might have some useful stuff though.

If you have questions about anything, let me know.

Also, look into some theory texts to expand your horizons.

This book written by Bruce Benward is a good place to start. It covers basic things like notation and properly writing music on paper, and then goes into scales, modes, intervals and chords, and all the way up to leading tones and song structure.

But the best thing it does is train you on how to analyze music and do what I did in the above comment. It also starts you on the path to counterpoint, which is kind of like a jigsaw puzzle method of writing short (like five measures) pieces.

This is the book we used for Theory I when I was getting my B.M.

This book is what I used for Theory I and II in college. You can probably find older editions too. They never change too much. It covers a lot and you can also learn how to write. Almost all of the examples are from classical music though. Try and get the CD that comes with it for the examples. Also, /r/musictheory might be a good resource for advice along the way. Also also, free printable sheet music is available here.