Since there are two people involved in the marriage, you will need two sets of documents, from both sides of the visa application process. In the first stage, when petitioning, the most important documents are those of the petitioner (US citizen), although already here you will need some documents from the K1 visa applicant. So, the first set of documents includes:
A completed I-129F petition form. This is to be completed and signed exclusively by the inviting party.
Evidence of face-to-face meetings and communication within the past two years. The key to successful visa approval is putting together a good package of these documents. Hotel, flight and other reservations, correspondence, photos of you together, gifts, etc. All documents are translated in Russian, photos (15-20 pieces are enough) are dated and signed.
Proof of US citizenship. This can be a copy of your birth certificate (on both sides), copy of your US passport (all pages, even blank ones), certificate of naturalization (on both sides) or other similar documents that you may need in case you lose your passport, for example.
Letter of intent to marry within 90 days. Written in free form in English, in two copies (from the groom and from the bride).
Documents proving that there are no barriers to the marriage. For example, if the applicant was previously married, a certificate of divorce or death certificate of the former spouse, in case of widowhood, is required.
Payment of a petition fee of $535.
2 passport-size photos of 5×5 cm (or 2×2 inches).
Form G-1145. This will enable the petitioner to be notified of the status of their case.
Once your petition is accepted for processing, you and your prospective spouse will receive a notice (Notice of Action 1) telling you the number assigned to your case. From that point on, you will have to wait for approval. When approval has occurred, you will receive a second notice (Notice of Action 2). Your documents will then go to the National Visa Centre (NVC), at which time you can prepare your application pack for your interview:
Documents confirming face-to-face meetings and communication with the fiancé/fiancée who petitioned for you. As with the previous kit, anything that credibly and clearly demonstrates that you’ve kept in touch and seen each other in the last two years will do.
Groom’s financial documents. This includes form I-134 (proof of financial support), tax form 1040, form W-2, bank statement, certificate from employer and/or other documents which confirm that income of the petitioner is more or equal to 125% of the living wage established in his region of residence.
Documents of medical examination. The cost of this service varies from $195 to $225 (depending on the age of the petitioner), and it is only carried out at two certified medical centres in Kiev. If you have underage children who you are taking with you, they must also undergo a medical examination.
Documents proving that there are no impediments to marriage. In the same way as for the American side, you will need to provide evidence of divorce or death of your former spouse, if you were previously married.
Evidence of the birth of minor children.
Military ID card. Required for male and female military personnel applying for fiancée / groom visa.
Payment of the consular fee for the visa ($265).
Arrangement for an interview at the US Embassy in Kiev.
2 passport-size photos 5×5 cm.
After a successful interview, you will receive your passport with your K-1 visa glued in it, and a yellow envelope that cannot be opened. You will need this envelope when you go through border control in the United States. Once you enter the United States, you have 90 days to marry the citizen who petitioned for you. And since the fiancée visa in the US is an immigration visa, you will be fully eligible to apply for residency, i.e. a green card, after you get married.