Do a search on the forum on Diatonic harmony - it will hopefully show you some threads that explain which notes and chords are in which key, and how to figure it out. As a start:
C Major:
Notes - C D E F G A B (and back to C again)
Modes - C Ionian, D Dorian, E Phrygian, F Lydian, G Mixolydian, A Aeolian, B Locrian.
Chords C Dm Em F G Am Bdim
The pattern is always the same. What you need to do is work this in reverse and there are clues... For instance, as you can see if you have a progression that includes Dm and Em there is only one place you find two minor chords a tone apart which means they would have to be degrees 2 and 3, and the key would have to be C Major. If the chords were Em and F#m the key would be D...
Do a forum search, and try 'Diatonic Harmony' in google - just take it slow - it's not rocket science, but you need to do a little at a time to avoid being overwhelmed by it... :cool:
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