If you’ve ever corresponded with someone in Messages before, you’ll probably see that person’s name in the left-side list of conversations. (Or, if you’ve got a lot of conversations listed there, you can use the search box above the list to find that person.) Click the name and then start typing into the message box. The “conversation” with that person continues, even if the last exchange was months or years ago.
To start chatting with someone you’ve never corresponded with before, click the Compose button () next to the search box. The To: blank appears.
As you type your buddy’s name into it, Messages sprouts a list of matching names from your Contacts. (If this person isn’t in your Contacts, see “The Buddy List” on The Buddy List.)
Some of them probably feature a blue speech bubble (). Those are people with iCloud accounts—with whom you can exchange iMessages.
An iMessage address can be either an email address or a phone number. Most people register both of those data bits as part of their iMessages setup, so that any message you send arrives on all your correspondent’s Apple gadgets at once.
You can also click the button to get a pop-up version of your Contacts, ...