Turning on iMessage in your iPhone requires that you go to Settings > Messages and switch on iMessage. Ensure that your phone has an active Wi-Fi or mobile connection. You can be asked to use your Apple ID. When turned on, your messages will be transmitted in the form of an iMessage to other Apple phones.
To enable iMessage on iPhone, open Settings, tap Messages, then toggle on iMessage. Ensure your device has internet access via Wi-Fi or cellular. Wait for activation; once complete, you can send and receive messages using your Apple ID.
To switch on iMessage in your iPhone, follow: Settings > Messages and switch iMessage to On. Enter your Apple Id in case it prompts you. Make sure your device can be connected to the internet (Wi-Fi or cellular). Upon activation, you are able to receive and send iMessages using your email or cell number.