Well, I guess the first thing you should start with in make a clothing line is a theme. Where are you going to draw your inspiration from in order to create clothes that reflect that particular inspiration. For example, Lagerfeld did a huge ocean inspired collection for spring. Secondly, a color story! How is your color story going to reflect your style as well as what the line is trying to say. You should look into Pantone's color archives and do a little color trend forecasting/research to see what's going to be hot this fall. Wines, aquas, clover (super light pink, not quite bubble gum), purple, and other dark, yet colorful colors will be popular this up coming season. Of course, taking on any business venture like that will be expensive, but if you have passion, drive, and determination to succeed, you will!
As for manufacturing expenses, you probably won't need to consider that quite yet because I doubt your demand will be high enough yet. I'm sure whatever you make will sell wonderfully! But, manufacturers usually require a minimum for orders since they have to create special machine inter-workings in order to accommodate whatever style/cut you are trying to create!
My friend just started a clothing line, Original Dope Designs, and she sells all of her clothing that she makes on Etsy.com ( she makes custom made, dip-dyed/studded cut offs), and she gets orders from all over the world! So there are multiple avenues to which you could begin your clothing line with. Gone are the days of needing your typical brick and mortar store! You should never let money discourage you from a dream. Where there is a will, there is a way! Best of luck to you! and I hope this answers your question!