Assuming the person works for you as an employee - any compensation for the service is generally considered wages - and you are required to withhold employment and income taxes.
If commissions are supplemental wages - you need to select the way to determine income tax withholding (that should be the same for all employees):
--- the commission amount is added to regular wages and withholding is determined based on W-4 filing
--- withhold from any supplemental wages including the commission payments at flat 25% rate.
Please see IRS publication 15 for more details and withholding tables - http://www.irs.gov/pub/irs-pdf/p15.pdf