If you have the money to open a clothing boutique, you have the money to have a hour consultation with a QUALIFIED accountant/accounting firm, and also with a QUALIFIED business attorney who has experience with businesses -- and not simply just incorporating them.
I strongly suggest you do that before proceeding. Depending on your state, there could be benefits to forming a LLC, or incorporating, or there may be penalties to doing such (such as a franchise tax) which would make being a sole proprietorship the obvious choice.
From my experience, go as a sole prop for now, and make sure you purchase umbrella insurance for you and the business, but a LLC may, I repeat may be an ok alternative. It depends on your state laws regarding the filing process for them, and if there's a franchise tax.