Dear Vicky, there was a time when Models had to be beautiful/handsome/fair, etc. Now times have changed and curvy, dark and aged are also walking the ramp. So there is no set criteria to become a model. Yes, one should have a pleasing personality and a good physique because one would endorse products. There are Institutions that offer modelling classes and it depends solely on the city you re living in. And Fashion designing is an interrelated field but has got nothing to do with modelling. When you join any NIFT for a Fashion Designing course, you straight away don't design clothes for celebrities and models. That is not fashion designing, You learn the intrinsics of designing in various fields like textile, handloom, knitwear, etc. You learn these basics and put them to creative use and design clothes. And initially for you to start off, you will have to work as an intern under a designer who has established himself/herself. You learn the art and then slowly start on your own by increasing your network. For a Celebrity  to wear your clothes, they should be out of the world or you should have a reputation.