how can I attach an external microphone to my iMac?

What year model is your iMac?

Identify your iMac model - Apple Support

With an older Mac that has a headphone and audio input port, you can use the audio input port.

With a newer Mac that has a single port, you need to use a headsets that has a built in microphone.

In either case, your best bet is to just get and use an external USB microphone.

see > https://ioshacker.com/accessories/best-usb-mics-for-mac-users