Which choices are essential?

So many choices!

Prism's nonlinear regression dialog has seven tabs, and can seem overwhelming at first. But you don't have to learn about all the choices when you first fit a curve.

If your goal is to fit a standard curve from which to interpolate unknown values...

... you can ignore most of the choices that Prism offers. You do need to pick a model, but you should judge the adequacy of the model visually. If it goes through the points without wiggling too much, it is fine for interpolation.

If your goal is to fit a model to determine best-fit parameters....

... then choosing a model is the essential first step. You should not expect a computer (or a software company's tech support) to choose a model for you. Choosing a model, and deciding which parameters should be constrained and which should be shared among data sets, is a scientific decision that is fundamental to analyzing your data. The other choices are useful, but can be put aside when you are first learning curve fitting..