An unmarried dependent student must file a tax return if his or her earned or unearned income exceeds certain limits. To find these limits, refer to Dependents under Who Must File in Publication 501, Exemptions, Standard Deduction, and Filing Information. You can also refer to Do I Need to File a Tax Return? to see if your income requires you to file.
Even if you don't have to file a federal income tax return, you should file if you can get money back (for example, you had federal income tax withheld from your pay or you qualify for a refundable tax credit). See Who Should File in Publication 501 for more examples.