Your 2008 iMac is capable of utilizing an external monitor in one of two modes:
- Extended Desktop
- Mirror
What you want is "mirror" mode.
To do this you need to be able to get access to System Preferences (via the Apple Menu)...and this is the difficult part when you have a dead internal display (can't see the menu's and windows) to do this.
I've done this a couple times in the past...and it can be a VERY difficult process. You're basically "fishing in the dark" on the dead screen.
One thing to check that may help. Even though the display looks "dead"...it may not be...it may just be that the display backlighting is dead. Take a flashlight ("torch" if you are from the UK)...and shine it on the display (with the computer on) at a sharp angle...and see if you can see the desktop.
If you can...then you can maneuver your mouse to open System Preferences...make adjustments in the "Display" System Preference. Wham-Bamm...you can now do everything from the external monitor!
If your iMacs internal display is truly dead...then you may need to try the tedious "fishing" method.
Good luck,
- Nick