PC
Press the Windows logo key 
+PrtScn. The file is saved to the Screenshots folder in your Pictures folder. You can also us the Snipping Tool which is available in all versions of Windows Vista, 7, and 8 except the Starter and Basic editions.
Mac
To take a screenshot with your Mac, Command + Shift + 3 and then release all keys to captuer the whole screen, or press Command + Shift + 4 and press down and drag the mouse over the area you'd like to capture.
iPhone/iPad
To take a screenshot with your iPhone or iPod Touch, press and hold the sleep/wake button and then click the Home button.
Android
Take a Screenshot with Android 4.0 (Ice Cream Sandwich) just press and hold the Volume Down and Power buttons at the same time.
CS:GO Case Simulator