How do you record your screen on High Sierra

For still image screenshots:

The Command-Shift-5 option came in with Mojave and the new Screenshot app.

In High Sierra you can do Command-Shift-3 for the whole screen area,

or Command-Shift-4 to select an area, a crosshair will appear on screen

move it to where you want to take the screenshot then click and drag to

to select the area you want to take a screenshot.

If you want to see the Screenshot keyboard shortcuts open

System Preferences> Keyboard> Shortcuts> Screenshots.

If you want to do a screen recording, i.e. a moving image

then use the Quicktime app as mentioned above.

Open QuicktimePlayer.