You will need to use Schedule C Business Income and Expenses to report your income from freelance work. Since you were not their employee, you are considered to be self-employed through your own business. You can enter your income as 'cash' without needing any tax form showing the amount. You can use your own records to determine the amount of income that you had.
Take a look at the following TurboTax help articles to get started:
What Is a Schedule C IRS form?
Where do I enter Schedule C?