Once you've set up your business in TurboTax, click the Business tab and then click Start/Update next to Profit or Loss from Business. Click Edit next to the name of your business. Scroll down to the Business Expenses section to select the appropriate category for your expense.
If you haven't set up your business yet, follow the instructions to do so. You'll then be able to enter your expenses in the Federal Taxes section.
Basic, Deluxe, and Premier versions
These don't have the Business tab. Instead, click the Federal Taxes tab, select Wages & Income, then I'll choose what I work on.
Near the bottom of the Income Summary screen, click Start/Update next to Business Income and Expenses (Sch C). After you've entered your business info and income, we'll ask about your expenses.