Here are some resources that will help you with music theory.

MusicTheory.net - This website starts with the absolute basics of music notation and moves through various scales, chords, and chord progressions. It's a self-paced website that you can use completely for free.

HookTheory - a website that catalogs various chord progressions used in popular music. Of particular interest is their chord progression trends page which will help you design your own chord progressions.

ChordChord - a website that auto-generates chord progressions based on various genres. Great tool for trying out chord progressions before you commit to them. If you use the chord progression generator tool, it will often suggest chords that are more complex than our basic 3-note chords, but you can always water them down to their basic versions (take note of the roman numerals below the actual chord name!)