How to get mathematical model from a data set with MATLAB

Question

I have some values imported from excel about the annual sell of a product, with 3 var, month, price, sold items I have seen how to plot a chart using curve fitting tool but I need to define a math expression that can describe the relation between these variables. How can I do this in matlab? Is there any tool that can achieve this automatically. if you look at the chart should exist a way to use an approximation that can mime the behavior(maybe with spline functions or n-point-hermit), is enough one approximated expression of the model that for the 3 var replicate a similar chart if plotted.