The easiest way to take screenshot of a computer screen is to use Print Screen or Print Screen/SysRq key of your keyboard. It is generally located at the top row of the keyboard right after the F12 key. When you press the key a snapshot of the computer screen gets copied into the clipboard. Now you can paste & save the image in any graphics editing program like MS-Paint or in a word processing program.
If you want to take the screenshot of only the current window then press Alt + Print Screen keys. Isn’t it quite simple