Turn on imessage - how?

If you're coming back to iPhone from Android, or leaving for Android, there's a catch the other way round: iMessage keeps your number registered and texts from iPhone users can vanish into it. Turn iMessage off on the iPhone before you switch, or use Apple's deregister page if the old phone is already gone.

Settings > Apps > Messages on newer iOS (just Settings > Messages on older), flip iMessage on and sign in with your Apple Account if it asks. If it sits on "waiting for activation", give it time, it can take up to a day. It needs to send a text to activate your number so you need a working SIM with SMS. Under Send & Receive you pick which number and emails it uses.