To print screen on a Mac there are three main options:

  1. Press and hold the following buttons until you hear a camera shutter sound : Command+Shift+3
  2. Press and hold the following buttons until the cursor changes to a crosshair.  Then drag the cross hair around what you want to ‘print’ : Command+Shift+4
  3. Press and hold the following buttons until the cursor changes to a crosshair.  Press the space bar.  The cursor changes to a Camera.  Now you can photograph any window in any application : Command+Shift+4

All of these methods create an image file which is saved to the Desktop and named picture1 picture2 etc.  You can then use that image file for whatever your purposes are.