In order to make your payroll program work, you must have the Social Security number of each employee you pay.
You should get this information from the employee's W-4, since you need that form to accurately withhold federal and state taxes from your employees' paychecks.
The Form W-4. You need to get a signed IRS Form W-4 from each employee, not only to get the employee's Social Security number, but also so that you know how many allowances an employee is claiming for purposes of federal and state withholding.
You should give all new employees a Form W-4 to complete as soon as they come in for their first day of work, if not before. A good time to take care of this and other necessary paperwork is during employee orientation. Employees should also complete a new W-4 if they get married or divorced, have a baby or gain (or lose) another dependent, or want to change their withholding amounts for any other reason.
On the federal Form W-4, line number 5 on the Employee Withholding Allowance Certificate portion will tell you how many allowances the employee is claiming. This number will determine how much you should withhold from employees' checks for payroll taxes.
Once you've got that information, but before you can determine how much to
withhold and which method is best, you must determine how
often state law requires you to pay employees and establish
a pay period.