There is a ton of good info out there, both in books and online. One thing you can do, early on, that will help immeasurably, if you haven't already, is to learn/memorize all 12 major scales, assign numbers to each and every scale degree, then get to the point where you can instantly identify every scale degree by number in all 12 major scales. For example, in the key of C, the '5' is G, the '7' is B, the b7* is Bb, the '3' is E. In the key of F#, the '4' is B, the '3' is A#, the b3 is A, the '5' is C#, etc. *Start by learning the diatonic notes (the notes in the scale), then it will be easy to identify the chromatic notes (b3, #5, etc.).
Once you're able to do this for every tone in every key, you will have a good foundation for further study.
It gives you a solid reference point. So, for example, to derive a Cmajor7 chord, you can use the formula 1 3 5 7 (C E G B); Cmin7 would be 1 b3 5 b7 (C Eb G Bb); C7 = 1 3 5 b7, and so on. That is only one small example, but the numerical system is how a lot of this is described and will only make sense when you know it in 12 keys.
Roman numerals are used to name chords in reference to a key center. G7 would be V7 in the key of C, and the G7 chord would be spelled 1 3 5 b7 (G B D F) based on the root, G. Of course, you can also have that same G7 chord in other keys; it could be the I7 in a G blues, the IV7 in a D blues, or a bVII7 in the key of A, or VI7 in the key of Bb, etc.
If that's already starting to get confusing, it won't be once you have all those scale degrees down.
p.s. Edited because I kept thinking of things to add; I could go on and on and confuse the issue totally.
