Want to turn off iMessage? You're not alone. Many people move from Android to iOS, of course, but others make the move in reverse.
And if you're moving to a non-Apple device you will want to turn off iMessage so you don't have problems with lost texts. Remember that on iOS, blue bubbles are for iMessages, green are for SMS text messages.
The reason texts get lost is that when you change phone, Apple continues to treat iMessages sent to you from iPhone, iPad and Mac users as, well, iMessages - they aren't automatically delivered as texts. That means they get lost rather than being delivered.
If you still have your iPhone you can turn off iMessage easily. Make sure it has the appropriate SIM card in then by going to Settings, tapping on Messages and turning iMessage off. Also in Settings, tap FaceTime and turn FaceTime off. And that's it.
If you no longer have your iPhone, you can deregister iMessage at Apple.com by entering your phone number. Apple will then send you a confirmation code that you'll need to enter into the site. Don't forget to click the drop-down to tell the site what country you're currently in.
If you're worried about someone getting your messages in iOS, make sure you've turned on Read Receipts. iOS gives you the option to manage Read Receipts individually. Tap the tiny arrow at the top near your contact's name, then hit the "i". There you can toggle the Send Read Receipts option on or off.
By the way, if you are having trouble sending your iMessages over your mobile data you can get iOS 12 to send the message as a SMS instead. Go to Settings > Messages and toggle the Send as SMS switch.