Go to Settings > Messages, where you can:
Turn iMessage on or off
Notify others when you’ve read their messages
Specify phone numbers, Apple IDs, and email addresses to use with Messages
Set SMS and MMS options
Show the Subject field
Block unwanted messages
Set how long to keep messages
Filter unknown senders
Manage the expiration of audio messages and video messages created within Messages (audio or video attachments created outside of Messages are kept until you delete them manually)
Manage notifications for messages.
Set the alert sound for incoming text messages.