Most startups fail because they were not sufficiently capitalized. Remember, you can't just put all of that $20K into the company, you have to live on it, too. And you won't be able to do this unless you take out massive loans, since you don't have any other funds. What are you going to do next year, and the year after that? Most companies that survive don't start turning a profit until they have been in business for at least 5 years.
Go to college, take business courses, and find a part time job in the clothing trade, even if this is just selling clothes at Old Navy.