Many went to design school where they were taught what their next step should be. For most designers it's working under an established designer learning the business inside and out -behind the seams as it were. The reality is that few ever get to the level of valentino or armani. You need to work on being the best, most creative and most innovative that you can possibly be, and to have a unique vision that no one else has. The Huge designers all have at least on uniquely innovative thing that made them stand out form the crowd. For Valentino it was his creative vision, for Armani it was how he redefined tailoring and structure of suits. But before you can be at the top, you do need to spend some time at the bottom learning the tools and tricks of the trade.
Give us some more information.
i was going to make a joke about MLP:FIM Rarity, but rakesh is right
Search fashiondesigncourse in youtube and watch all the videos. I am also fashion designing student and following all the step from that videos. Good luck.