iMessage is likely already enabled on your phone,
but you may have it turned off to conserve data.
Here's how to check and enable it if it is turned off.
Unlock your iPhone and tap on the settings icon
to open settings.
In settings, scroll down until you find "Messages"
and tap on it.
In the middle of your screen,
you'll see an on-off slider for iMessage.
If the slider is to the right and green, iMessage is on.
If not, tapping it will turn it green,
and iMessage will then be enabled.
You will now be able to use iMessage
to send and receive messages on your iPhone.