Go to Settings and tap Messages.
Turn on iMessage and tap “Use your Apple ID for iMessage.”
Enter your Apple ID and password.
Tap Sign In.
Your phone number and all email addresses associated with your Apple ID will be displayed.
Note: Your 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, then tap Next.
Under “Start new conversations from,” choose the email address or phone number you’d like to use to send outgoing messages.
Note: If you’ve already set up other devices for Messages, they’ll alert you that a new device has been set up for Messages.
Exchange words. Or photos. Or videos.
iMessage lets you send messages back and forth with anyone on iPad, iPhone, iPod touch, or a Mac running Mountain Lion. Send photos, videos, locations, and contacts, too. If you have more than one Apple device, iMessage keeps the conversation going across all of them. And you can text and send photos and videos via MMS to other mobile phones over cellular networks.
Text one. Text all.
Send a message to one person or to many friends at once — along with photos, videos, or links you like. When someone replies, everyone sees it. With more people you’ll get more texts, but remember, it’s iMessage, so every text is free over Wi-Fi.*
Blue is best.
On iPhone, when your text bubble is blue instead of green, you’ll know you’re using iMessage instead of SMS. You’ll know that you can get a quicker response. You’ll know that the text session is free. You’ll know it’s been sent to all your recipients’ Apple devices. And you’ll know they received the text. SMS texters will be green with envy.