If you're using an Android phone (or tablet), most of the time you'll have to press the volume down and power buttons at the same time to take a screenshot. Alternatively, if you bring up the app switcher, you should be able to find the screenshot button on screen.
If you're playing on a retro gaming handheld, it will depend on the brand. For example, if you're using a Retroid handheld, swiping left from the right side of the screen will bring up the screenshot button.