I would like to purchase a microphone for use with a Roland 60 electric guitar amp. Does it have to be a dual impedance or do I need to purchase something to improve the volume level?
I assume you mean the AC-60? This has a mic/line input with phantom power so should work direct with most dynamic or condensor microphones. If using a condensor mic, make sure it doesn't need more than 10mA current as this is the maximum it can provide according to the specs:
Other than that you shouldn't need anything else - apart from a cable and a mic stand :-)