The U.S. State Department website details the documents needed for a B1/B2 visa or a O/P visa.
B1/B2 documentation: https://travel.state.gov/content/visas/en/visit/visitor.html#documentation
O/P documentation: https://travel.state.gov/content/travel/en/us-visas/employment/temporary-worker-visas.html
You should also consider contacting a lawyer for additional assistance. Please see the Additional Resources FAQ for a list of some attorneys/agencies that SXSW artists have used in the past, however, you are free to choose any lawyer you wish.