You can read the instructions for the Affidavit of Support (I-864 http://www.uscis.gov/i-864) and the Poverty Guidelines (I-864P http://www.uscis.gov/i-864p).
US department of Health and Human Services publishes a list of the qualifying income levels based upon the size of the household on an annual basis. The list was recently published in the US federal register. It would be easy for you to check for this information online. If you need additional information, please feel free to consider enlisting the services of a qualified immigration law professional.
USCIS uses Poverty Guidelines for determining whether you meet the income requirements or need a joint sponsor. You can go to the following website to find out this information: http://www.uscis.gov/sites/default/files/files/form/i-864p.pdf Good Luck.
In order to determine the amount of documentable income required, you may go to this link, http://www.uscis.gov/i-864p and review the amount required for the applicable family size. If you are not thoroughly familiar with the process, such as using that link to determine the applicable amount or determining what documentation of income will be sufficient, then it would be wise to engage an immigration attorney to assist you - the cost is often significantly less than people expect.
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