There's really full advice, written by pros, on AppleSupport. You could find it by googling.
In short, hold down the top of the scrollwheel and the centre button until the Apple appears on the iPod's screen. Then shift your grip so that you continue to hold down the centre button, but now press down at the bottom of the scrollwheel.
That should force you into Disk Mode.
To access diagnostic, do the same thing, but shift your grip to hold down the left hand side of the scrollwheel.