Download the IRS Schedule C Instructions from the IRS website (see Resources). In the 2011 version, the codes begin on Page C-10.
Find the general category that your business falls under. The categories are listed alphabetically. For example, if you have a coal mining operation, you would find the "Mining" category.
Find the subcategory that best represents what services your business performs and you will find the business activity code you need to report. In this example, you would find that the code for coal mining is 212110.