You fill out a tax return on a 1040, 1040A or 1040EZ form to calculate your taxable income, then refer to a tax table or schedule to calculate your tax liability. Comparing that to the amount already paid in gives you what you owe additional, or what will be refunded to you.

If you get the 1040 booklet, it explains it all right in there. Or you can go to the IRS website (www.irs.gov).
How about looking at your past returns & see what has changed so far this year? I think that’ll be the easiest