Estimated tax payments are due to the IRS on June 15 for the second quarter. (If you use a fiscal year calendar, then your second quarter payments are due by the 15th day of the 6th month of your fiscal tax year.) Use the 2012 tax tables to estimate your income and determine an approximate payment.
Small business owners are required to make estimated tax payments because U.S. income tax is a “pay as you go” system. You can’t just wait until next April and pay everything at once.
Remember there is a sort of tax withholding safe harbor that states that if, over the course of the tax year, you pay 100 percent of the amount of taxes you owed the previous year, you will not be liable for an under-withholding penalty. However, you will still have to pay any amount you owe come April of next year.