A reflectance calculation typically consists of converting data to radiance (by applying a gain and offset or scale factor) and then reflectance (by performing an atmospheric correction). You will need to know something about the current 'raw' data. Is it in DN that first need to have a gain/offset applied? If you don't know you will need to contact the data provider. If the data is already in radiance, then you can perform an atmospheric correction using a model such as FLAASH or Quac, or use one of the relative reflectance tools in ENVI such as Dark Subtraction, Internal Average reflectance, log residuals. If you have field collected data you can use Empirical Line calibration.