The fiancé in the relationship will need these documents;
A completed Form DS-160- print the confirmation page of the form and bring it to the interview
A valid passport that can be used to travel from the UK to the US
A birth certificate
Any police certificates from the UK and any other countries they have lived in during the past for at least 6 months, since the age of 16
Any divorce or death certificates from previous marriages
Medical examination
Evidence of financial support (Form I-134)
Evidence of your relationship with the US citizen fiancé
Two 2×2 photographs that meet requirements