Follow these steps:
If you paired an Apple Watch with your iPhone, unpair your Apple Watch.
Back up your device.
Sign out of iCloud and the iTunes & App Store.
If you're using iOS 10.3 or later, tap Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.
If you're using iOS 10.2 or earlier, tap Settings > iCloud > Sign Out. Tap Sign Out again, then tap Delete from My [device] and enter your Apple ID password. Then go to Settings > iTunes & App Store > Apple ID > Sign Out.
Go back to Settings and tap General > Reset > Erase All Content and Settings. If you turned on Find My iPhone, you might need to enter your Apple ID and password.
If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].
If you're switching to a non-Apple phone, deregister iMessage.
Contact your carrier for help transferring service to a new owner. If you aren't using a SIM card with your device, you can contact them to get help transferring service to the new owner.