On your tax return's Schedule C, Line B, you need to enter the Principal Business or Professional Activity Code that corresponds to your business's activities. There is a list of these 6-digit codes at the end of the Schedule C instructions. (HTML version here, or you can look at the last two pages of the PDF version.) The directions at the top of this list reads:
Select the category that best describes your primary business activity (for example, Real Estate). Then select the activity that best identifies the principal source of your sales or receipts (for example, real estate agent). Now find the six-digit code assigned to this activity (for example, 531210, the code for offices of real estate agents and brokers) and enter it on Schedule C or C-EZ, line B.
Although there are a lot of codes, it is entirely possible that you won't find one that exactly matches what you do. The directions say to pick the "best" one that you can. First, pick the broad category. You haven't specified your business, but let's say that you are a freelance programmer (a common occupation for Stack Exchange users). The category you decide is best might be "Professional, Scientific, & Technical Services." There are several subcategories and activity codes under this, and you might find one that fits your business. However, if you don't, at the end of most categories, there is an "Other" code. For our example, there is code 541990, which is "All other professional, scientific, & technical services."
If you can't even find a broad category that describes your business, there is the last code in the list: 999999, which is for "Unclassified establishments (unable to classify)."