How you take a screenshot on an iOS device depends on the model. For screenshot purposes, it all comes down to whether or not your device has a Home button.
With a Home button
Simply press and hold the Sleep/Wake button and then press the Home button
You should see a visual confirmation and may hear an audio confirmation depending on your sound settings
Once a screenshot is captured, it will be automatically saved to your Photos
Without a Home button
Simply press and hold the Sleep/Wake button and then press the Volume Up button
You should see a visual confirmation and may hear an audio confirmation depending on your sound settings
Once a screenshot is captured, it will be automatically saved to your Photos