Strouhal number

Hi Dear all,
I need some help from any one. I just want to know how to use the matlab function FFT for calculating the Strouhal number. We have already the drag and lift coefficient data, uniform inflow velocity and we know the diameter of the cylinder. Like St=f*D/U. So can any one tell me please how to find the shedding frequency f and then the strouhal number. If some one can explain with an example that will be realy appretiated.
Thanks all and bye for now.
Best Regads