All non-immigrant visa applicants applying for visas at the U.S. Embassy or Consulates are required to complete the DS-160 online application form. You are required to fill out this online application form right before you make your consulate appointment.
Please find a step-by-step online tutorial right here. The US Embassy in London has a YouTube on how to complete the DS 160 (https://www.youtube.com/watch?v=DbLWi3de1hA)
Also, this link will help you with frequently asked questions on completing the DS 160.
