Windows phone provides facility to take screenshot in device. In windows different- different version have different steps to take screenshot. Basically you have need two button for take screenshot. If your device is windows 8.1 then Volume button and Power button, if windows version is 8 then Windows button and Power button.
Here I am creating a sample of screen shot
1. In Windows 8.1
First sure that your phone unlock or not then press Power button and Volume button simultaneously. After follow this step screenshot will save on picture tab.
2. In Windows 8
Windows Phone 8 devices you can take screenshots by simply pressing the power and the Windows button. Press both button simultaneously.
Screenshot will be saved automatically and can found in Picture hub - Screenshot Picture Album.