The procedure for taking a screenshot on Windows Phone depends on its version. Until version 8, this is not possible, so you have to download a third-party application from the app store in your smartphone.
To take a screenshot, you need to simultaneously press the key and the button to turn on the smartphone until a click sound, which means that the screenshot has been generated and will be saved in the gallery.
To take a screenshot in WP 10 or WP 10 Mobile, you must simultaneously hold down the power and volume up keys.