Will it be difficult for me to file for green card and how may we prove financial support since we both are currently unemployed?

Your question states that both you, the future Beneficiary, an F-1 (OPT) student, and your boyfriend (fianc?), the future Petitioner, are currently unemployed, as you are both students. Note that there are other important legal considerations regarding the Adjustment of Status process (I-130 Petition for Alien Relative/I-485 Application to Adjust Status to Permanent Residence), including vetting any inadmissibility issues, that should be discussed with an immigration attorney. However, to address your question about the I-864 Affidavit of Support alone, it is possible to apply for Adjustment of Status even if the Petitioner's current income does not qualify (i.e. is not 125% or more of the poverty line for your household size) if: a) he can qualify using his assets; b) he can qualify using your joint (post-marital) assets, though some conditions apply, if these are foreign assets; or c) if you obtain a qualifying joint sponsor to file an I-864 on your behalf. Finally, though your question initially stated you are unemployed, you mentioned that your current status is F-1 OPT. Should you obtain OPT employment, you could use your income to qualify for the I-864A, Contract Between Sponsor and Household Member.

You will need a sponsor with income to file an affidavit of support above the poverty guidelines.

First, congratulations on getting married and completing such a high level of education. That's great! To determine any difficulty you may have, a more complete history is needed. For example, have you maintained lawful status consistently? Was your most recent entry lawful? Have you had any immigration proceedings? Has anyone ever tried to petition for you before? Have you or your boyfriend ever been married? Has he ever petitioned for a spouse before? What is your current means of support? Is there a family member that would be willing to "co-sponsor" you? There are other questions worth asking, too, but these examples give you an idea of the kind of information I would want if I were evaluating your case. Generally, your age difference could raise a red flag, but you would have an opportunity to prove the validity of your relationship. Document gathering would be key. For the financial support issue, you'll need to explore your options for sponsors since you and your boyfriend are not employed. It's probably a good idea for you to consult with an attorney so you can get an idea of what to expect. At least if you speak with an attorney, you'll have a better idea of the process and requirements. Good luck!

The process is simple. After you get married, your boyfriend has to file a Form I-130 petition for you. Since a visa number is immediately available for spouses of U.S. citizens, you may file your Form I-485 application to adjust status at the same time the Form I-130 petition is filed. If your boyfriend has no income, you will need a co-sponsor who is either a U.S. citizen or lawful permanent resident, can meet the applicable income requirement for sponsoring you, and is willing to execute a Form I-864 affidavit of support on your behalf. The co-sponsor need not be related to you or your husband.