Go to ustraveldocs.com and click on the immigrant Visa link on the left side of the page. It explains the next steps. This is from their website:
U.S. Embassy Appointment
Immigrant Visa applicants who have been notified by the National Visa Center (NVC) or the Consular Section and K visa (fiancé/fiancée) that their case is ready for interview should go here to create a profile and schedule an appointment to visit the Visa Application Center (VAC) in the 10-day period prior to your interview at the U.S. Embassy
Before your appointment at the U.S. Embassy, you will need to gather the required documents; including the medical exam, police certificate, and birth and marriage certificates. Please bring your appointment letter and plan to arrive no more than 15 minutes your appointment time. Please read about security restrictions at the U.S. Embassy that describes what you can and cannot bring with you to your interview.