I'm sorry i didn't read the above post, but how i'm going to explain it is.....find the relative chord!

For example, "Bell Bottom Blues" by eric clapton

verse is Key of C

last line in the verse ends with F G

two verses, then into the Chorus, which is modulated to key of A. How do you get to A?

Look at the last chords played, its a C Major Walk up, into a Major 6th chord, A.

IV V VIMaj
F G A

For the chorus, the A becomes the I chord, etc.

Get Hal Leonard's "Harmony and Theory", its excellent.