You are not required to enter business expenses if you have none to claim. You can still report your self-employed income on Schedule C without including any expenses.
However, you must be using the Deluxe version of TurboTax Online at a minimum (or any of the desktop versions of TurboTax) since Self-Employment tax will be calculated based on your self-employed income.
See the following information to get started:
Where do I enter Schedule C?