To sponsor your family member, submit a United States Citizenship and Immigration Services (USCIS) Form I-130. Each person you sponsor needs a separate Form I-130. You can submit the form online or by mail.
The sponsor does not have to prove the relationship to you (if you are the one being sponsored) but would have to give information of the spouse that was part of the joint filing including the US citizenship or legal residency. The marriage certificate would prove the relationship to the sponsor.
No, you cannot have a random person sponsor you it has to be an immediate relative. If you arrived here legally, you can marry a US citizen and have them file for you.
If you have a joint sponsor, they must also complete Form I-864. If you are using the income of other household members to qualify, then each household member who is accepting legal responsibility for supporting your relative must complete a separate Form I-864A, Contract Between Sponsor and Household Member.
To upload additional financial documents or add a joint sponsor, you will start on the Affidavit of Support Tab. From this page you can add a new sponsor or a household member. When adding or updating financial documents, you have to first choose the sponsor whose documents you are uploading from the drop-down list.
Joint sponsor needs to provide the following documents: Proof of US citizenship or permanent resident status. Federal tax return or IRS tax transcript for the most recent year. All W-2 and/or 1099 form(s) for the most recent year.