By pressing the PRINT SCREEN button.
The key may appear differently depending on the manufacturer of your Windows keyboard. The standard position would be above the navigation keys (that are positioned to the left of the numeral keys). The button may also have the acronym Prt Sc imprinted on it.
The sole function of this key is to take a snapshot or picture of your computer screen and copy it to the clipboard (the clipboard is an invisible holding area for content that is copied). Its the same thing as if you copied something from a document and it is now on your clipboard waiting to be pasted in its final destination