There are different ways to create a screenshot or screen capture depending on what operating system you have.  

On Windows Vista and Windows 7, by far the easiest way to do this is with the Snipping Tool.  

• Go to the Start menu and search for Snipping Tool. Click on that option.
• Click the arrow next to the New button and select Free-form Snip,Rectangular Snip, Window Snip, or Full-screen Snip from the list.
• Select the area of your screen that you want to capture by putting the crosshairs in one corner and clicking and dragging the cursor diagonally to the opposite corner.
• This will open up a window asking you where to save it and what you want to name it.
  

On Macintosh, a screenshot can be taken by holding down the Command, Shift, and 3 keys together.  It automatically saves to your desktop.

To just capture a portion of the screen, use Command, Shift, and 4 together.

On a Chromebook, to get the whole screen, click the CTRL and "Switch Window" button   at the same time.  

To capture just a portion of the screen, hold down the CTRL and SHIFT keys and then click the "Switch Window "button.