There are two ways to take a screenshot on your iPhone.
The first way is to press and hold the Home button and the Sleep/Wake button at the same time. You will see the screen flash white and hear a camera shutter sound, indicating that a screenshot has been taken.
The second way is to use the AssistiveTouch feature. To do this, go to Settings > General > Accessibility > AssistiveTouch and toggle AssistiveTouch on. A white dot will appear on your screen. Tap this dot and then tap Device > More > Screenshot.
Again, you will see the screen flash white and hear the camera shutter sound to indicate that a screenshot has been taken.