Fit of a data set to different probability distributions

I am working on evaluating the fit of a data set to different probability distributions. After estimating the fit parameters, I want to create a Q-Q plot comparing my observations with the theoretical data. However, I don't know which theoretical value to assign to which observed value. For example, what is the theoretical value for the minimum value of my observations? I can't find a reference for this. I would appreciate any help.