I'm not an expert but I'm guessing you will have to include proof from the IRS or your own signed statement as to why you don't have to file taxes (if that is the case)
Payment advice/stubs of your current non taxable income, 6 months preferably.
Make some notes in the NVC section to explain all his to them
Joint sponsor can be a backup. As I recently found out, as long as you supply all required financial documents, i.e. proof of income and last tax return (or reason of no filing) then it should come back okay