Several things, not all and not in order:
Where are your nearest competitors, do you want to compete head on or open in an area outside their coverage?
What is the maximum you can afford to spend each month and for how long if you make no profit (most new businesses lose money the first 6 months and are out of business the first year).
Will you have employees, what will you pay them? Do you know the added costs with a payroll? Workmen’s comp, matched SS, etc.
How will the accounting be done?
Who will your suppliers be, will they carry credit?
Do you need a loan to start? The lender will want a detailed projection of costs and income.
Will your lease include utilities or will they be separate?
Will the space need to be remodeled? Lighting and display for fashions is VERY important? If the space has been vacant for sometime, perhaps the landlord will do some of the remodeling in advance?
Is there a provision in the lease to get out of it if you lose money and must close doors? Can the lease be tied to profit at all? What if you grow and need to get a larger space?
Ask police department about robberies, thefts, break-ins relative to area(s) looking at. Do you need a security system? Does it need to have 24-hour monitoring? Have a button that may be pushed for a silent alarm and police call-out.
How will you accept payments, what are costs associated with credit card payments?
Do you need storage space? Will the storage be suitable for protecting the items from damage?
Are there changing rooms and bathrooms? You do not want potential cusotmers leaving becauase they have to go elsewhere to use the bathroom.
Is there room for an office and can you monitor the retail space if you are in the office?