Easiest way would be to live in NYC, LA, MIA, or SF; then start testing with the local agencies, shooting actual fashion models. During said shoots you should be networking with creatives (ie. hair, makeup, wardrobe, designers...the last 2 being very important). In addition, have a positive attitude and a good reputation, as well as (eventually) a strong portfolio of work. Use that time to get friendly with the agents/bookers (sometimes they can refer work/opportunities to you), since they are a direct line to all the clients. After a while you can start submitting to magazines to get more credits/tearsheets to your name. Eventually you should be getting paid work from designers/clothing lines.
Wash, rinse, repeat...until you're booking big jobs or campaigns. For some, it'll never happen. For others, it might take 20 years. Or, if you're lucky...it might take only a couple years.
One thing to remember, for the majority of people: in the fashion industry your talent and skill isn't as important as how well you can "play the game"...in other words there's a lot of politics involved, and knowing who to schmooze, and what to do/not do and when.
A clothing line is unlikely to help you, unless you're interning for a major brand and you just happen to meet the right person. Even then, you aren't going to learn fashion photography on the job working for a clothing line, you'll be doing dummy-work like packing boxes or getting coffee. The only photographers they'd likely have would be in-house photographers for their ecommerce stuff, which isn't really "fashion" in the sense that most people think of it. I've yet to see one that needs interns/assistants for the photographer either, but YMMV
ETA: This assumes you live in the US. Not 100% up to date on what the market is like in Europe for example, but the obvious ones are Paris, and Milan, and even London in the UK. I think Melbourne or Sydney (can't remember which) had work in Australia as well from what models have told me.