Once iMessage has verified your email address, you're brought to the Messages settings screen. Here, you can enable or disable iMessage altogether; turn on Read Receipts, which lets others see when you've read their message; or allow the Messages app to send your text as an SMS if iMessage can't get through. This applies to the iPhone only. Disabling this setting prevents your phone from automatically reverting to SMS--and racking up texting charges--should iMessage fail. It doesn't, however, disable SMS entirely.)

Other settings let you choose the email address or addresses you'd like people to be able to reach you at. (You can associate multiple addresses.) There's also an SMS/MMS section, which allows you to enable or disable MMS Messaging (iPhone only), Group MMS Messaging (iPhone only), Show Subject Field, and Character Count (iPhone only).

To start writing a new message in iMessage, tap the new message icon in the upper right corner. In the To field, type the email address or iPhone mobile number of the person you want to message. If you've never tried to send a message to this person before, the app will automatically spot-check the address when you finish typing, to ensure that it is iMessage-enabled.

If it is, you'll see the email address, phone number, or (if the address is in your contacts) person's name light up in a blue bubble, acknowledging that you can go ahead and compose your message. In the future, when you type that person's name or email address, you'll see a little blue chat bubble next to it in the drop-down menu, signifying that this contact is iMessage-enabled.