OK, just to throw a few more confusions into the ring.

What key is it in? Generally it will depend on the start and end chords. Then you have to consider what chords are used in the rest of the piece.

Example: A song starts with C chord but uses G and D chords. That'll probably be in G then........

A useful little table

CFG Basic chords in C
DGA
EAB
FBbC
GCD
ADE
BEF#

You can also "modulate" (i.e. change key) within a piece. A simple example is where a straight blues in A moves up to B for a dramatic effect. You might come back to the home key but not always.

Now how do you work out what scale you want?

Every chord is made up of a number of notes that have a particular relationship to each other. Let's look at the simple "major triad".

C - the notes in the scale of C are C, D, E, F, G, A, B, C

A major triad is made up of the 1st 3rd and 5th notes of the scale

Therefore C is C, E and G and if you're in the key of C you'll be wanting notes from the C scale. You might also want to consider notes from the F and G scales when the chords move.

I think this is probably way too much information and I don't have the time now to explain it properly. I'll try and get back in the next few weeks and do it again :D

Cheers

Dick