They require two things from you:
1. Evidence that the marriage was in good faith.
Financial commingling documents: bank account statements, shared bills, 401k, life insurance, etc.
Shared residence documents: ID's or mail with same address, lease or deed, etc.
Shared plans documents: will, power of attorney, health care directive, etc.
Shared life documents: boarding passes from trips, cards from family, photographs together, etc.
2. Evidence that the petitioner -you- is a US citizen or an LPR.
Copy of passport or citizenship certificate if US citizen
Copy of green card if LPR