Cortana is one of the main new feature in the upcoming Windows Phone 8.1 update. But it is limited to US Windows Phone user for now and Microsoft is planning to expand it over the course of this year. If you downloaded Windows Phone 8.1 preview today and want to try out Cortana, here is how you can enable it.
1) Go to Settings and change your region and language to US English.
2) Also set interface language to US English.
Restart the phone once and you will be able enjoy Cortana now!