Taking a screenshot is the fastest way to explain a problem. Showing the error instead of saying "it gives an error like this" usually cuts the conversation in half.
Every system has its own shortcut, and they are all different from one another. The way to do it on all four systems is written out separately below; just find the step that applies to you and follow it.
The whole screen or a selection
In most cases the whole screen is unnecessary. Capturing only the relevant part makes things easier for the other person and keeps you from sharing something private by accident — don't forget that your open tabs, your notifications and your file names end up in the image too.
Before you share
If the image contains an e-mail address, a phone number, an address or account details, scribble over them. Most phones and computers offer a drawing tool right after the screenshot is taken.