carbon copy

i have backed up my entire computer using the option 'carbon copy' (i believe it was). it's all on my external drive.
so i hear there's a way to reboot your system from an external disc.
if so, i've no idea how.
please help?

Dont quote me but if you go into System Pref/ Startup disk just choose your external if it shows

Or hold down the option key at bootup and then select your startup drive.