If you have employees on your payroll, your tax situation is significantly more complex. You'll have to keep track of payroll taxes, employee withholding, Social Security, Medicare, unemployment, disability, and worker's comp, to name a few. All of these require separate calculations and timelines for payment. These rules are very strict and complex, and if you're not in the payroll business, hire an accountant or a payroll service to keep yourself legal and punctual with all these payments.
All employees must fill out a federal W-4 form and a Form I-9 from the Immigration and Naturalization Service. You should also check with your state's office of taxation to find out about state income tax, unemployment, and worker's compensation, as well as what forms, if any, need to be filed.
If you use independent contractors and pay them more than $600 per year, you must file a Form 1099 with the IRS and send one to the person you paid. You can get these forms from your accountant or from tax officials. The Form 1099 reports to the government that this person was paid as a non-employee. This gives you the right to deduct those payments as an expense and tells the IRS to look for that money as income on your non-employee's Schedule C form.
Your tax preparer should provide you with forms and envelopes for making your estimated payments, or you can order these forms directly from the IRS by calling 800-829-3676.