Using lsqcurvefit for curve fitting

Hey, could you tell how to use the lsqcurvefit fuction. I got an actual stress and calculated stress. I am trying to optimise 3 constants in my expression for calcuated stress to match with the actual stress. The expression in MATLAB I got is x = lsqcurvefit(fun,x0,xdata,ydata). I believe xdata and ydata corresponds to the measured stress. It would be great if anyone could tell me what I got to write into fun, x0 and what x is, in the above expression

[KSSV]

fun would be a function which you feel fits your data....check the examples at the documentation. http://in.mathworks.com/help/optim/ug/lsqcurvefit.html