If you are receiving cash income, you do own a business. You are your own business. You will need to report your income on Schedule C as self-employment.
You will file your income and expenses on Schedule C describing the type of work that you do for the income you are receiving. Enter your income as 'cash' income when you are working on the Schedule C.
See the TurboTax articles below for more guidance:
Am I considered self-employed?
How do I report income from self-employment?