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

If your goal is to interpolate from a standard curve, 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. Note that Prism 6 offers a simplified curve fitting analysis for interpolation.

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

If your goal is to fit a model to determine parameter values, 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..