If your passport has no surname and only shows a name, you should enter your name for ESTA exactly as it appears on the passport.
In general, the full name shown on the passport should be entered in the Last Name field, and FNU, which means First Name Unknown, should be entered in the First/Given Name field.
Since the name details must match your passport exactly, please check the English name shown on your passport carefully before submitting the application.
For a smoother trip, please apply for ESTA before your departure.