Most of the music theory I learned was from the music fundamentals course for 1st year music majors at my university. Non-music majors like me were allowed to enroll.
Some basics to learn - this is not a comprehensive list btw:
What a quarter note is in a typical 4/4 beat and what the smaller beats are (eighth notes, dotted eighth notes, etc.)
Intervals from minor 2nd to octave
The major scale and its degrees
How to build chords from the major scale
Which chord of the major scale is the dominant and which is the tonic chord
How to get minor scales from the major scale
Some chords built on a minor scale (usually harmonic minor, sometimes melodic)
These basics lead to the ability to break down your favorite songs, which leads to further study of the underlying theory, and starting to understand why choosing scale 08 sounds like shit on some chord progression in your groovebox vs. scale 23 or whatever.
After university, most of the theory I picked up was off of jazz guitar and jazz keyboard courses. Once in a while i get something cool off of a Youtube channel like 8-bit Theory.