SIC stands for "Standard Industrial Classification." It is a system of classification of industries according to the type of activity the industry is engaged in. SIC codes, along with the more modern version known as NAICS (North American Industrial Classification System), are used by the United States government for statistical purposes.
Go to OSHA's (Occupational Safety & Health Administration) website page on SIC codes at http://www.osha.gov/pls/imis/sic_manual.html.
Scan through each of the Divisions ("A" through "J").
Click on the Major Group within a Division that best applies to your business.
Read the description of that Major Group once you've clicked on it, to be sure it fits your business.
Look for your SIC code next to the specific section of the Major Group you chose. For instance, if you chose "Major Group 82: Educational Services," the SIC code for an elementary school would be 8211.