There might be more depending on your situation, but here goes:
Documents for the US Citizen:
1. Proof of citizenship (one of the following):
a. Copy of birth certificate – if the US Citizen was born in the U.S.
b. Copy of all pages from the U.S. passport.
c. Certificate of Naturalization.
d. Report of birth abroad of a United States Citizen (form FS-240).
2. Birth certificate (even in not selected in 1 as a proof of citizenship).
a. If the US Citizen was not born in the U.S. - Birth certificate and a translation of the birth certificate if the birth certificate is not in English. See “How to translate documents into English” below.
3. Copy of all pages from the U.S. passport (even in not selected in 1 as a proof of citizenship)
4. Proof of current employment – at least 2 most recent pay stubs and a letter from your employer.
5. Proof of tax return including:
a. 2008 tax return (form 1040 or form 1040EZ that was submitted to the IRS).
b. W-2 for 2008.
c. Any 1099 form or any other form that shows income and was reported on the 2008 tax return.
d. Optional – 2007 and 2006 tax returns.
6. Proof of assets – if used as part of form I-864.
7. If was married before this marriage – proof that all previous marriages were terminated (copy of death, divorce or annulment certificate).
Documents for the alien spouse:
1. Copy of all pages from current passport.
2. If the visa that the alien used to enter the U.S. is in a separate passport – copy of all the pages from the passport with this visa.
3. Birth certificate and a translation of the birth certificate if the birth certificate is not in English. See “How to translate documents into English” below.
4. Copy of the I-94 (front and back).
5. If had a previous work authorization card (EAD card) – copy of the card, even if it is expired (front and back).
6. Any visa approval notices from the USCIS or the Department of Homeland Security, such as H1B approval notice, F1 visa documents, etc.
7. If was married before this marriage – proof that all previous marriages were terminated (copy of death, divorce or annulment certificate).
8. Only if using the alien spouse income as part of I-864:
a. Proof of tax return including:
i. 2008 tax return (form 1040 or form 1040EZ that was submitted to the IRS).
ii. W-2 for 2008.
iii. Any 1099 form or any other form that shows income and was reported on the 2008 tax return.
iv. Optional – 2007 and 2006 tax returns.
b. Proof of assets – if using the alien spouse’s assets as part of form I-864.
c. A letter from the employer stating that the source of income (job) will continue after the alien spouse will receive the adjustment of status (green card).
Documents for the couple:
1. Certificate of marriage.
2. Evidence of bona-fide marriage:
a. Documents that show co-mingling of financial resources – such as copies of joint bank account statements, joint credit card statement, check that has the names of both couple.
b. Document that show joint ownership of property – such as joint deed on a house or a joint ownership of a car.
c. Documents that show that the couple lives together – such as a bill from a utility company (electricity, cable, phone, water, etc.) that has both the couple names on it, or a rent agreement.