There are thousands, possibly millions, of books on how to start your own business. There are even quite a few related specifically to the fashion industry. Depending on what country you'd like to own a business in, there are various laws related to starting a business (with additional regulations for foreigners). Even within one country, the laws may vary from one region to another. Rather than address this whole process, I'd like to talk about a whole other issue altogether. It is a question you should be ready to ask yourself. The question is, are you ready to start your own business?
Perhaps some of the best advice I was ever given as an undergraduate was that even if you want to start your own business, it is better to work for someone else for at least a couple years first. That way, "you can make your mistakes on someone else's dollar". Furthermore, I'd like to point out that while you are doing so, you are also (hopefully) making some dollars. One of the major reasons businesses fail is due to lack of cash flow. Of course, this can be remedied by applying for loans and grants. The eligibility of such loans and grants is going to vary from place to place and person to person. However, you will require some form of capital in order to start a business. The scale and nature of the business must also be considered. It would make the most sense to find work in a company similar in nature to whatever it is you'd like to do.
Also, be sure to ask yourself, "Do I want to be in charge of my own business?" There are many headaches (usually including working long hours) that come with owning a business. It may not always pay as well as working in a larger company and/or for someone else. If money is the main motivator, it may be possible to make just as much with less effort working for someone else.