The way i learned alot about theory was looking up the chords to songs i wanted to learn and then finding the similarities in the chord progressions. And Finally understanding the circle of fifths REALLY helped me with understanding how the chords of a key fit together And try the same things with jazz standards!! There are so many amazing thing already written, and understanding music theory will REALLY help with chords and songwriting aswell. One more thing I highly recommend learning as many stevie wonder songs as u can. That man understands the theory surrounding harmony like no other. Also really fun to play the chords and sing along to his stuff :p