The easiest way to shut-down your iPad is from iPad settings - or from the Control Centre. If you are triggering a screenshot when pressing physical buttons, you are unintentionally using the hardware button sequence that will trigger this function.
To shut-down the iPad from settings:
Settings > General > Shut Down
To shut-down the iPad from the Control Centre, simply swipe downwards from the top-right corner of the screen to expose the Control Center, the tap the Shut Down button at top-right:

Be aware that routinely shutting down your iPad is generally unnecessary - unless you don't intend to use your iPad for many days. When the iPad is locked and otherwise idle, background housekeeping tasks will be running - such as retrieving email/notifications and automatic backup of your data to iCloud. When shut down, these tasks cannot run.