What do i have to consider to set the duty cycle and the ratio in the transformer in order to design a flyback converter? the input voltage are between 18-36 and the output is 3,3

Unless you are doing this as a research project, the answer is go buy what you need. There are lots of 24 V input dc-dc converters available off the shelf from companies like Emerson/Artesyn/Astec, Delta, Murata, Ericsson, TDK, Lineage Power/GE Energy, and Bel Power. If this is for a inclusion in a product, you will be far better off buying a proven design that is made in high volume.

And if you are a student doing this as a research project check out this paper in your library. It should answer your questions

"Taking Black Magic Out Of Flyback Converter Designs" Combined Proceedings of HFPC September 1996, pp. 311-321

the off period should be sufficient to denergise your magnetic circuit., i.e. the transformer. Normally duty cycle should be maximum of 50%. For safe side I had taken 45%.