First you need to load the email library.
Do this either in the controller file that will be sending the email:
Or load it globally in the autoload.php file in the config folder:
$autoload['libraries'] = array('email');
While you're there, you may want to load the email helper if you want to use some of CodeIgniter's built in shortcuts:
$autoload['helper'] = array('email');
The email helper can be loaded in the Controller file in a similar way to the email library: