How to do non-linear regression without a model?

Hi. I have 5 input variables. 1 is a categorical variable, but the others are numeric. I have one response variable, and I want to do a non-linear regression to find the equation of best fit in Matlab. How do I start? I have no idea what the model of the equation would look like, so I'm not sure if I can use fitnlm().

[Gideon Maasz]

I am sitting with exactly the same issue. I have 7 predictor variables and 1 response variable in 2 arrays respectively. No idea what the model should look like or how to approach this. I did play around with the fitnlm function, but am getting stuck where I have to supply a 'modelfunc' as I don't have 1. I have all toolboxes so that won't be an issue.