A business license grants you the right to operate a business within the city. Start by contacting your city and/or municipality’s business license department. You will fill out an application and upon doing so, the zoning department will check that your area is zoned for your business purpose (this will not happen in a day). If you are starting a business out of your home, investigate thoroughly. Residential neighborhoods tend to have stricter zonging laws. It does not mean you will not be granted a license, you just may need to file a variance.
City of Chicago: https://www.chicago.gov/city/en/depts/bacp/provdrs/bus.html