Some students have asked me why I don't explain theory all that much. No reason in particular. Theory is important to learn, but not as important as some believe.
Having said that, there are some things all students should know. The first and most important in my book are chord progessions.
The most important think you have to understand as far as diatonic harmony is concerned is that there are certain "tensions" that can be created using the l - lV - and V chords from a particular key.
For example, in the key of C Major, the 3 most important chords (iin terms of creating these tensions) are the root chord - C Major, the fourth chord up, F Major, and finally, the fifth chord up from the scale - G Major or G 7.
While many, many books have been written about theory, nothing substitutes actually trying it out on your own keyboard!
To prove the "tension creating aspect" of all this, play a G 7 chord then a C Major chord. Hear the way the tension created by the G 7 chord resolves completely when you play the C Major chord?
Anyway, I found this amazing resource that explains theory a lot better than I can. It's called Ricci Adam's Musictheory. net and is an excellent and FREE resource that should be helpful to you. Click here to get it.