I've got a 20 year old Cannondale mountainbike (model # scratched off long ago). It's been used as a daily commuter for a few years and for a couple long trips. I've been told that the original chainring and cassette need to be replaced but I don't know anything about sizing and compatibility. How do I go about choosing replacement parts?
Assuming Shimano components.... You shouldn't have any difficulty with compatibility. However, as noted above, you need to find out if you have a cassette/freehub or a freewheel.
At 20 years old I imagine you're looking at a freehub...