There are several popular tablet platforms (Android, iOS-iPad, and Windows Phone), but the procedure for taking a screenshot is different.
1. Simultaneously hold down the volume down button and the tablet power button (top or side).
2. Release both buttons quickly. A white flash is a signal that a screenshot has been taken.
On the latest iPad versions, there is no Home button and to take a screenshot you need to hold down the top button and the volume up button.
On older iPad versions, you need to hold down the Home button and the button on the top of the tablet to take a screenshot.
If you have a tablet running Windows Phone 8, hold down the key and the power button of the smartphone until the sound of the latch.
If your tablet has Windows Phone 10, then simultaneously hold down the volume up and power keys.