If you do not know the primary SIC code for your business, you can search this site to find the 4 digit SIC code that most fits your primary business.
https://www.osha.gov/pls/imis/sic_manual.html

 

Or, as an alternative, If you do not know the primary NAICS code for your business, you can search this site to find the 6 digit NAICS code that most fits your primary business.
https://www.census.gov/cgi-bin/sssd/naics/naicsrch?chart=2017