how would i connect an external mic to an Imac?

You will need a USB mic. There are many available to choose among. Audio Technica, Sennheiser, Rode, Shure, Mackie, Logitech, even Monoprice all have good mics. Prices range from about $35 to as high as you want to go.

Here are a few suggestions that are under $100:

• Mackie EM91**
• Audio-Technica ATR2100X-USB
• Audio-Technica AT2005USB
• Rode NT-USB
• Logitech Blue Snowball iCE
• Monoprice Dark Matter Sentry

Alternatively, you could use any analog microphone but in that case you would also need a USB audio interface like a Focusrite Scarlett, Presonus Studio, Motu, Behringer U-phoria, etc. There are many different models in each line. But doing it that way will usually be more expensive than a simple USB mic.

In all cases you would need to change the sound input setting in your iMac, to point to the mic or audio interface. See Change the sound input settings on Mac - Apple Support

The iMac has a built-in microphone – so I presume that the reason you want an external one is for better sound quality (e.g., for making podcasts).

If that's the case, you may want to check reviews on the Internet to see which microphones people recommend. Some microphones have USB interfaces and could plug directly into your iMac (or a dock). Others have regular audio outputs (of some kind), and you could attach those using an appropriate USB "sound card."

Change the sound input settings on Mac - Apple Support

And if you need a Mic, USB...

https://www.amazon.com/USB-Microphone-Mac/s?k=USB+Microphone+for+Mac

Nice info, thanks! :)