Look at your keyboard, top row to the right of the F12 key. You should see the "Print screen" key. Press it and a copy of the screen is put on the clipboard.
If you use Alt+PrintScreen, you get a shot of the in-focus window or application.
Next, open any graphic application, like the very good and free IrfanView, and press Ctrl+V.
You can now save that image in any image format you want, like PNG ot JPG.
Alternatively, there are many screen shot applications to be found. Some free, some costing a LOT. You can take a look on download.com, filehippo.com, tucows.com, 01net.com, and many others.
Electro, December 2013