If you file a tax return for any reason, you must report all your income, no matter how small.
If self-employment is your only job, you must file if your net income (profit after expenses) is $400 or more. However, note that if you receive a 1099-MISC or 1099-NEC and don't include it on a tax return, the IRS will send you a letter asking for more information and assessing taxes. Even though you might have zero net profit, it may be necessary to file that officially so the IRS has the records of your expenses and doesn't think you owe taxes. The IRS does not have to give you credit for expenses unless you prove them.
