Having dealt with a number of such companies in the past, and being with Jay, Michael, and some others now, I can tell you that it is a tough route.
Jay is by far one of the balliest dudes I know. He funded and built his own workshop, which is no mean financial or technical feat in and of itself.
If you are interested in technical work, you are going to need such a workshop. Otherwise, you are going to be stuck working with mainly plastisol, and maybe waterbased, inks, and a commercial printer, who may be great, or absolutely crappy.
If you are not doing a tee-shirt or hoodie line, you are going to have to learn to be highly proficient with a sewing machine, pronto. Also, you are going to need to find a reliable source of fabric. If you get a bunch of orders, and then run out of fabric, and can't get some (of exactly the same - retailers do not like substitutions - they see it as bait and switch, unless you upgrade from say, wool to cashmere, like Yoko Devereaux's manufacturer did for one piece for the F/W 2004/2005 season) you are shit out of luck.
It is also important to have exposure - i.e. people know you, know your product, know your face. Unless you have backing (i.e. are rich) and have good representation, you are going to have to do a lot of self-promotion, repping yourself and trying to get your foot in the door. It is nearly impossible to sell over the phone. For this reason, it is a huge advantage to be in L.A. or in NYC. You need to be at trade shows (these are expensive, btw. POOL, easily the cheapest of the shows, charges at about 4-5K a booth,) but lots of retailers don't go to these shows, and even when they do, your line is going to be one of several thousand they will pass by. You will be on the phone a lot, asking for appointments with buyers. Be prepared for a shitload of rejection. Have contingency plans (are you willing to sell on consignment?)
Anyway, not to be negative, but there are plenty of "designers" who work as waiters. If you are sure that FIT or Parsons and paying your dues is not for you (you would learn a lot,) be prepared for a long grind and hope that you get lucky.