iMessage lets you send and receive SMS-like text messages, and MMS-style multimedia messages just like a phone. Instead of a just a phone number, however, it uses your email address, and it only works with other Apple devices, like other iPads, iPhones, iPod touches, and Macs. Before you can start using messages, and sending texts, photos, videos, voice memos, locations, and contact cards, however, you need to set it up.
If you set up your device using iCloud there's a change iMessage is already set up as well, and good to go. If not, we'll show you how to get started, and get messaging, quickly.
That's it. If you have never linked your iMessage account to your Mac, just click the + sign instead and add your account.
Once iMessage activates you can send and receive texts and multimedia messages with other Apple users just like SMS and MMS! These instructions work for times when you need to re-activate iMessage as well due to a network error. Just toggle it on and off and you should be good to go!
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: