It looks like you mean "I want to design screen prints", rather than design clothes. This is actually pretty easy. Find a local screen-printer, and they'll probably be able to sell you some American Apparel (or Alternative Apparel) blank shirts at wholesale rates, and print your designs onto them. You might make about $10 per garment. It's really, really, really not worth the effort of having tshirts or tanks custom made.
If you want to truly design clothes; each step of this process requires hiring someone, since you probably don't know the skills or industry or have any contacts. Unfortunately, the newer you are at this game, the more people you have to hire, and the less money you can make. In fact, it's almost impossible to make any money at all as a new label unless you do most of the work yourself, and this involves experience and formal schooling, mostly.
You'll need to pay someone to sketch out your designs, and convert those into patterns. You might try doing this yourself, but this is a little like building a house without an architect. Or building a website without knowing HTML.
Next, you will have some samples made up, have the patterns modified so the fit is good (bigger labels actually have people called fit models). You'll need to adjust the pattern for all sizes you want to sell (XS-XXL).
It's best to have your samples made locally so you aren't waiting 6 weeks between revisions. Fit is everything! Once you have a few complete sample lines, you can take them around to local retail buyers and try to sell your line based on these.
You'll have to make sales before you actually have your garments made in bulk, unless you have a bunch of money to throw down the drain. Many people go to the Magic or Project trade shows in Vegas. You rent an (expensive) booth, make appointments with all the buyers, and on the days of the show, they come and make orders. (You don't know the buyers? You'll need to hire an agent) They won't pay you yet.
If you do make orders, now you can get your garments made in bulk in China, or wherever, and hopefully they can be shipped back by the time the orders are due (i.e. in time for the next season).
Something that takes half an hour to make will cost you several dollars in the US or tens of cents in China.
Unless you have a good contact at the other end, the first few runs will be horribly wrong, because people don't pay attention, particularly on new/small clients with low revenue. This happens no matter where you have things made.
Finally, you get your garments shipped, and then you send them off to be dyed, screen-printed, tagged, and packaged.
Then you deliver them to the places you sold to, and then 3 months later they pay you. Note that this is the first time you see any money in this whole process..!