Did you get a W2 from your employer? He might not have taken enough withholding (box 2) out of your checks to cover the tax on your income. Or you have a penalty for something like not having health insurance or the penalty for a IRA or 401k early withdrawal.
Did you file a return last year? You should print out your return and compare it side by side, line by line to last year and see where the differences are. Maybe you'll spot something you left out or entered wrong.
What lines look wrong on your return? You can (and should) preview your return. Go to TAX TOOLS on the Left side.
(You might have to click a little to the left of "Tax Tools" to make it open
Then click on TOOLS
Then View Tax Summary (first link in the Tools Center box)
Then Preview My 1040 on the Left side again
Then Click Back near the top of your screen to exit the preview screen.