Loss/storage modulii for spring mass damper system, how to?

Hi everyone, I am trying to find the effect of varying the spring constant and the viscous damping coefficient have on the system. In order to do this, I model the system using matlab (I can post the code of needed) and I am suppose to FFT to find phase lag and the modulii and see the effect of varying the parameters above have on making the system more solid like or more fluid like. I am not sure how to accomplish this and google have not been a good source. Can someone please either post a useful example or direct me where to go? Thank you