Go to Settings and tap Messages.
Turn on iMessage and tap “Use your Apple ID for iMessage.”
Note: iPhone screens shown. On an iPad and iPod touch, you won’t see the screen on the right, you’ll go straight to the Sign In screen below.
Enter your Apple ID and password.
Learn more about Apple ID
Tap “Sign In”
Any phone number and all email addresses associated with your Apple ID will be displayed.
Note: A phone number will be displayed only if you are setting up your iPhone, or if you are setting up another iOS device after you’ve set up your iPhone
Tap Send & Receive.
Choose which email addresses and phone numbers you want people to use to send you messages.
Under “Start new conversations from,” choose the email address or phone number you’d like to use to send outgoing messages.
If you’ve already set up other devices for Messages, they’ll alert you that a new device has been set up for Messages.
Note: iPhone screens shown. iPad and iPod touch screens have the same options shown but differ in appearance.