I suspect you're more worried about the plan than the actual business, perhaps because it's a school assignment. If that's the case, then do a web search for "Boutique clothing business" and see what turns up. The google search turns up a 2.7 million different hits. Be imaginative, search on "clothing store consultant" and "clothing store sourcing" and see what comes up that.
You've asked this question at the entrepreneur.com website, and on that same website for about the cost of a nice lunch you can buy a book called "Start Your Own Clothing Store." That might be a good idea.
Also, use the telephone and your network of friends. Who do you know that knows somebody who has done this? Buy that person lunch and ask him or her your questions. Or buy dinner and ask to be able to call frequently with questions.
And if you don't have anybody like that, find clothing stores from a distance, away from where you want to start, and call them on the telephone and explain your problem and keep calling until you find somebody who can talk to you. What your asking for is information you really need to have before you start a business.
For websites on business planning, start with my site at www.timberry.com and follow the links there to other sites that have more information. That includes entrepreneur.com and www.bplans.com.