Here's a small guide on how to take a screenshot:
Press and hold the power and volume-down buttons together. There is a slight knack to this action: press the power button too soon and the display will switch off; press the volume-down button too soon and your screenshot may be marred by an on-screen volume slider. After a few seconds the screen will flash, and a notification will appear in the drop-down bar at the top of the screen, giving you options to delete, share or edit the image. You can also access the screenshot from your Gallery.
If your device has a physical home button, as is the case with the Samsung Galaxy S6, you may find there is a different method for taking a screenshot. On the Galaxy S6 you press and hold the power and home buttons together.