If you use a Mac, it's easy to take pictures of either the whole screen, or a part of it. You just need to press three keys at the same time.
take a picture of the entire screen
Command-Shift-3 will grab the entire screen. You'll hear a camera-shutter sound effect. When you go to your desktop, you'll find a new picture file (called Picture 1.png -- if you do it a second time, you'll find there's also a Picture 2.png).
select part of the screen
Command-Shift-4 will turn your cursor into a cross-hair. Click and drag to select a region of the screen, then release the mouse button. Again, you'll hear the shutter sound effect. As before, the picture will be saved to your desktop
note
You can press and hold the keys one at a time in the order shown. So - press and hold the "Command" key down, then press and hold the "Shift" key, then press either "3" or "4"