On an Apple computer the easiest method is to use Quicktime Player. Open Quicktime and navigate to the menu File>New Audio Recording. Next to the recording button there is an arrow downwards, click on this to select your input. If you don’t have any external device then just use your internal microphone.
If you have an external device, which for a desktop computer you probably will need, make sure you have all the necessary software/drivers installed before opening Quicktime. Follow the same steps and select your external device(s).
You can use also Voice Memos App, which works more or less the same as Quicktime, or you can also a program like Garage Band which might give you more options. Check the video bellow for some basic instructions.
On Windows you can use the Voice Recorder App. Check out this guide on how to install this app. If you can’t find it on your windows machine and you really need it, check the video bellow to see how to install and work with it.
You can also find apps for Android (Google) and iOS (Apple) which record only audio. This helps in terms of space size and if you don’t really need to send a video, you might want to use one of the available audio recording apps.
Check this guide to choose the right app.