Thanks to Simon Miles 1972 for this question.
NOTE: No-one ever taught me any theory, I just picked it up along the way.
Four simple (?) aspects to know :-

1. NOTES:
A, A#, B,C,C#,D,D#,E,F,F#,G,G# - and back to A !
Hint: Notes go note to "sharp" (#~) when going up, and flat (b) when going down - Crazy but that's the way it is.
Why no E# or B# ?? (or Fb or Cb?) Good question!
This is music theory - not common logical sense.

2. SCALES: (standard major scale)
Just using the basic major scale : Tone and semitone (step and half step) - from your root (or keynote)
tone, tone, semitone, Tone, Tone Tone semitone.

3. CHORDS : i.e. three or more notes played together.
Major chords use the 1st, 3rd and 5th notes in the scale.
Minor chords use the 1st, flattened third, and 5th note of the scale.

4. Chord progressions : Can by just two chords, or tree , four, five etc., i.e. one for each note in the scale (and sometimes ones that aren't!

So Simon, that's yer very basic music theory!
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