The formula is a little bit more complicated than that. It would be:
Payment = P*(I/12) / (1-(1+I/12)^(-n))
Where P is the principal loan amount, I is the APR interest rate, and n is the number of periods.
The best way to figure out payments is to use a financial calculator or you can use Excel. The formula for use in Excel would be
where P=principal, I=Interest per period, n=number of periods, FV=final value.