How come I owe taxes

If you have completely entered all of your income and withholdings, you would owe taxes because you did not have enough withheld and you did not make enough estimated payments to cover your tax liability. 

This usually happens when you have more than one job during the year.

Ex. You are single and you made $12,000 at one job.  The employer withheld your taxes based on an annual salary of $12,000, meaning you would have had $0 federal taxes withheld as your taxable income after your standard deduction would be $0.  You then have a second job and make $18,000.  But, this employer would only withhold taxes on $18,000 which means they would only withhold about $580 for the year.  Problem is, your total income for the year would be $30,000 and giving you taxable income of $17,800 and a tax liability of $1,942. Since you only had $580 withheld, you would owe the difference of $1,362

Other reasons that you may owe taxes are you were self-employed and did not make estimated payments during the year or you haven't changed your W4 withholdings after a life change that affected your tax breaks.

You can use TurboTax to fill out a new W4.  You will find this under the Federal section under Other Tax Situations section.