Hi Mert,
The function newff is obsoleted. In order to create a feedforward network, you can use some other function like feedforwardnet or fitnet. In either case, you need to design an optimized neural network for your data to get the best performance. Try looking at the existing examples in the documentation of this function and start designing your network. There is no hard and fast rule here to get the optimized results. You have to understand your data, visualize it and then design your network based on the data.
I hope this helps.
Cheers
