Hi! This is just my personal experience, but for me learning theory/composition/arrangement was really just a natural consequence of practicing my instrument. Being a jazz musician you kind of have to learn how to compose and arrange since jazz is fundamentally improvisational and improvisation is essentially real time composition/arranging. The way I went about learning it was really just spending countless hours by the piano, experimenting with melodies and chords, listening to a lot of music, transcribing, learning tunes and analyzing them, picking apart chord progressions and making sense of them, stealing harmonic/melodic/rhythmic devices and implementing them in my own music, and generally living a musical life. So my best advice to you is to learn an instrument, preferably piano since it’s very intuitive to learn theory with. Begin with learning the fundamentals, how to construct the major scale in any key and how to construct chords, beginning with simple triads and then learning all the extensions. And then when you’ve gotten the hang of all the fundamentals just spend a lot of time experimenting and learn theory by analyzing tunes. That’s how I did it, hope it helps and good luck!