Being young and a student doesn't matter. For SE self employment tax - if you have a net profit (after expenses) of $400 or more you will pay 15.3% SE Tax on 92.35% of your net profit in addition to your regular income tax on it. If you owe too much on your tax return you will have an underpayment penalty. Look at your tax return. Which lines don't you understand? Did you enter all your expenses on schedule C? Are you a dependent on your parents return?
The Self Employment tax on 20,000 should only be like $2,826 on 1040 line 23. The underpayment penalty is on line 38.
