If you're asked to fill out Form W-9 (Request for Taxpayer Identification Number and Certification), it's because the business or person hiring you needs your name, address, and Social Security or tax identification number for payroll and tax reporting purposes. This form does not have any income or expenses reported on it.
You would fill out the form and return it to the person who requested that you complete it. You should then receive a Form W-2, 1099-MISC,etc from the person/company, you would then use that form(s) to file your taxes.
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