If the w-2 matches your pay stubs (no tax was withheld) then just enter it as-is and proceed. You may not owe tax if your income was too low.
Or, if you owe a tax bill, it may be that your W-4 was filled out incorrectly by you, or was processed incorrectly by your employer. Use the IRS web site to help submit a new W-4.
https://www.irs.gov/individuals/tax-withholding-estimator
You can get into problems if you have more than one job; you might not have tax taken out of the lower paying job even though your combined income is enough to owe tax. When you have more than one job, you need to make adjustments on the W-4, the IRS web calculator should help.
