First buy hosting, domain name and after that go with the design and after the development last post content.
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I actually own a large ecommerce develomemt company and we have a lot of really good resources on our site. I'm not trying to spam a link, but there are a lot of things you need to think andweve written articles to help people like yourself. You'll need to think or everything from hosting to a cart and an SSL. Then how you're going to market the site. Look at http://www.outerboxdesign.com/web_design_articles/ and check out the blog, too.