Your question is very minimalist. You need to do everything:)
But here is where you should start:
Decide on what clothes you want to sell (style: new, second hand, hand
made, workers clothes, casual and you know the rest;: age group:
toddlers, teenagers etc; gender: homme, femme or both ) and then write
a strong business plan (search on line for examples of business plans
for clothes shops). When you got your business plan go to your local
business board and they will give you further advice and direction.
Writing a good business plan is the key to you getting money for the
start up. Research your target group, competitors and locations well.
When you`ve written your business plan consider having a consultation
with a lawyer to understand the legal side of the business because when
you take a grant or loan you need to be aware of all aspects.
Good graphic design for the good of your business.