How to use imessage to send messages after iphone 5s sends messages by SMS?

open iMessage and tap "Use your Apple ID for iMessage".

enter an account to log in.

your personal phone number and all email addresses associated with your Apple ID will be displayed.

please note: your phone number will only be displayed when you set up your iPhone or continue to set up another iOS device after setting up your iPhone.

How to send and receive messages

Select the email address and phone number you want others to use to send you messages, and then tap Next.

under "Start new conversations from", select the email address or phone number that you want to use to send information out.

please note: if you have set up other devices to use information, they will prompt you that a new device has been set up with information function.