If you are planning on operating your business under a name rather than your own, you will need to apply for an Assumed Name / DBA through the County Clerk's Office. Call 618-277-6600 ext-2363.
St. Clair County does not issue business licenses. If your business is located within a municipality, you will need to consult that municipality. If your business is located in an unincorporated area, a business license is not required.