Deciding to actually take this seriously during a little work break...
Two critical competencies first: you need to be able to draw (meaning not only drafting, but then finishing the design in Illustrator or whatever industry-specific applications there may be), and you need to be able to build a mock-up of the design (you don't have to be a tailor per se, but you need to be able to craft a functional garment.)
That suggests a few educational pathways: architecture, fashion design (obviously), an art degree specializing in illustration, etc.
You should have *solid* knowledge of the industry, an ability to monitor trends, and a knack for identifying market needs and how to target a niche (for a beautiful example of this, read up on the development of Spanx). Fashion is moving towards a vertical-integration retail model, so you'd want to understand that as well.
That's for being a *designer*; if you intend to actually launch a brand and be an owner, then obviously there's a suite of competencies involved in business management you'd need in addition to design.
Fashion is pretty networking-intensive as well, so you'd definitely want to hit trade shows and so on.
Be advised that like the fine arts, probably only 5% of fashion design majors actually wind up working the field, because it's intensely competitive; you always need to have a plan to remain relevant and seen (in advertising parlance, you need to gain and maintain a high Q score.) If it were me, I would focus on designing clothing lines with high brand value and groom them for acquisition. Rinse/repeat.
(Source: I have previously owned a company in the fashion space.)