Originally Posted by: LeedoggHow do you find the chords of a given key? Is that where the circle of 5ths thing comes into play?

Each note in a scale can form a triad. Take A minor. Starting with A, you form a triad. To form a triad, start with A, skip the next note (B), C, skip the next note (D), E. You have A C and E. Which forms an A minor triad. If you do this with each note, you get.

A C E - A minor (i)
B D F - B diminished (iio)
C E G - C major (III)
D F A - D minor (iv)
E G B - E minor (v)
F A C - F major (VI)
G B D - G major (VII)

So the chords in the A minor key are:
Am Bdim Cmaj Dm Em Fmaj G maj

Thats how you get the chord for a key.

All minor keys are the same.

i iio III iv v VI VII

Although the minor key has a much better scale for coming up with chords. The harmonic minor scale.

Creates this: i iio III+ iv V VI viio