In TurboTax Self Employed, if you enter into your tax return that you have started, you will want to select the Income & Expenses tab at the top of the screen. Once there, you should have the option of selecting the Self-employment Income and Expenses section to work on entering those items. TurboTax will then walk you through screens to enter your income and business expense information.
Here is a sample screenshot to find the "self-employment income and expense" item on the Income & Expenses tab.