You provide a written/signed statement (each of you) that you are intending to marry 90 days upon fiance/e entering US. You can have some preliminary places/ceremony details (full blown wedding at hotel X, court side wedding in County Court Y, etc). Just don't book anything if it means $. Assume 9-11 months end to end and you would have approximate dates.
Ok, you have enough time to prepare then..
Just make sure you saw/met each other within 2 years before filing the application..
Passport Entry/Exit Stamps (copy), Airline Boarding Pass, Copies of Debit/Credit Card transactions made while visiting fiancé(e) country, hotels under your name, etc.. are some samples of evidence that they would be looking for.. Anything that would prove you met each other within 2 years..
Pictures together, itinerary, etc.. are secondary evidences that you can also have..
letter of intent to marry from you and fiancé, signed and dated
G-325A for you and fiancé, signed and dated
USC birth certificate long form (copy)
Cenomar and NSO birth certificate for you
Proof on ongoing Relationship - other visits, calls, chats, txt msgs, gifts, e-mails, remittances, letters, cards, etc..
hmm.. I probably missed something.. But, just Ask! Good Luck!!!!!