How do I get my iPhone to send messages using iMessage?
This process is automatic, given that you've set up iMessage.
For more information on how to set up iMessage, view our FAQ entitled "How do I setup iMessage?"
Once you have iMessage setup, all you have to do is send a message (whether text, picture, etc) as you normally would. If the recipient of the message you are sending has also setup iMessage on their end, the iPhone will use iMessage. If the recipient does not have iMessage setup, it will use traditional methods.