The way to take a screenshot of the screen depends on your version of Android, if it doesn't work, try the next option.
1. Simultaneously hold down the volume down button and the power button of the phone (top or side).
2. Release both buttons quickly. A white flash is a signal that a screenshot has been taken.
1. Simultaneously hold down the "Home" buttons and the power button of the phone (top or side).
2. A screenshot will appear on the screen.
If you have keys at the bottom of the screen, then this is your way.
1. Press the Home and Back buttons at the same time.
2. Wait for the beep, after which the screenshot has been created.
When using both the first and second methods, screenshots will be saved in the "Photos" application, in older versions of Android - in the "Gallery".
There are a whole bunch of such applications, just download the first free ones in Google Play search.