I took Theory I and II at a university and basically the material below i what they gave me (minus the Youtube channel)
Beginner Level Theory:
Tonal Harmony by Kostka (Buy the workbook as well - this is a separate book) (Buy an older version - way cheaper and same info)
The main things to focus on is circle of fifths, voice leading (avoiding parallel octaves and fifths), counterpoint, analysis, figured bass, roman numerals,intervals, interval inversion and chord construction... really, you can skip chapters later on in the book, but don't do it early in the book. Theory is much like math where as you miss one part and you're doomed to understanding anything past that.
Work on aural skills out of
a new approach to sight singing by Berkowitz
musictheory.net for aural skill mastery (kind of like flash cards except online)
Rick Beato - this channel may be a little advanced, but he does have simple stuff too.
That will carry you to Theory III if not further.
It's been years and I'm still working on this same material; music theory is a huge topic - you can get your PHD in music theory alone.
Also notice that this focuses on classical theory. If you are a jazzer, you still want to learn the classical theory first, then move to jazz theory.