I don't want to use iMessage on an iOS 11 phone. How do I turn it off?
Turn off iMessage by going to Settings>Messages and switching off iMessage. 

Then make sure "MMS Messagging" is turned on.

For more information check here:


  Deregister iMessage on your iPhone
  
  If you still have your iPhone, you can deregister iMessage with these steps:
  
  
  If you transferred your SIM card from your iPhone to a non-Apple phone, put it back in your iPhone.
  Make sure that you're connected to your cellular data network.
  Tap Settings > Messages and turn off iMessage.