How do you design clothes and sell them?

Are you also going to sew the clothes? I would try a local shop that perhaps you can put your clothes in on consignment. Consignment means you put the clothes in the shop, they pay you IF and when they sell them. It is a very competitive business and until you get a reputation it is very very difficult to break into.
Good Luck !

If you are trying to sell your designs to manufactors or to established lines, it is tough, and many will steal ideas when given a chance.
It is a matter of working up sketchs of your work, and presenting them to the companies.

If you mean design, make patterns, sew up the clothes and then sell them to the public, as noted there are some cosignment shops in many cities, there is the internet, set up a eBay store, my sister in law makes her living doing that exact thing.