You do not fill out a W-2. That is a document issued to you by your employer to show how much you were paid and how much tax they withheld.
https://turbotax.intuit.com/tax-tips/irs-tax-forms/what-is-a-w-2-form/L6VJbqWl5
I think you mean you need to fill out a W-4 to give to your employer to tell the employer how much to withhold from your paychecks.
https://www.irs.gov/individuals/tax-withholding-estimator
https://www.irs.gov/pub/irs-pdf/fw4.pdf
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