If you have a USB microphone, it’s easy to plug it into your computer. But if you have an analog one, intended to be used with anything other than a computer — say, a camera, sound system or voice recorder — the chances are that it may not work, unless your computer has a dedicated microphone socket.
My Macs have a headset socket, into which you can plug combined earbud/microphone combinations such as you might get with your phone. But it tends to think that anything else you plug in there is just a pair of headphones, and that you probably want to keep using the built-in mic on the machine.
I did some experiments to work out how to persuade it to use an external microphone. It’s not one of my slickest videos, but it should serve its purpose!
Update, a few days later:
Even though the above solution works, it’s almost certainly easier, if you don’t have a microphone socket on your computer, to use a USB audio adapter, like this one.
I was hesitant about this, because I wasn’t sure of the likely quality of the analog-to-digital converters in a cheap USB peripheral, but it turns out to work very well for normal use. That would be my recommendation now, if you have a spare USB socket!