Pick a niche, online clothing is extremely competitive, don't start until you have solid sourcing (I had a store fail once because I did not have a strong source of products). Start looking at keywords and estimated monthly search traffic for those keywords, (2-4 word keywords are best and less competitive. Compare that monthly search number to estimated number of competitors, etc. Look for niches that have good traffic + small to medium number of competitors.
For started, eBay is a good choice until you get your own store up and running. It gets you involved in the basics (customer service, inventory management, sourcing, etc) in a pretty protective environment that has built in traffic. Meanwhile, build your ecommerce store on a platform that is geared towards beginners. Remember eBay and hosted platforms are good for beginners until you get your feet wet. They are not your long term solution. Take the progression in steps.
Also, the days of putting up a simple web store and making money easily are over. There is too much competition. Customers expect information above and beyond product listings. Stuff like How-To's, blogs, buyers guides, media such as video, and high quality content will attract visitors to the content and then ultimately to your store. Focus on building a community. Use Twitter, Facebook, etc to build followers. Contribute by giving them free, useful content. Become known as an expert in your area. Do this and word of mouth marketing will start occurring. People are still hesitant with merchants they have not heard of before. The above tips will build credibility. It takes time, but once you have an established community of followers, you will start seeing your sales take off. Meanwhile you can also supplement with PPC (pay per click advertising like Google or Yahoo), but choose your terms carefully, or your conversion won't be high enough to cover your advertising budget.
Learn everything you can about ecommerce. Buy books, visit websites, forums and ask questions. Check out sites like www.basicsofecommerce,com, www.practicalecommerce.com and others. They are reputable, and geared towards beginners. Subscribe to blogs. Stay away from any form of marketing crap that looks like a long sales letter. Those are trash.
Get a valid vendors license and if you can, form an LLC or Corporation. In addition to legal protection (although clothing is low risk), you will get more credibility with the real wholesalers (the ones who ask you for a vendors license, etc)... You will separate yourself from the many who just appear as casual parttimers and small online stores that most distributors want nothing to do with.
Its tough but can be done. I have been doing it since 2001 and just now passed my 500k per year in sales mark, across 3 websites. And I still sell on Amazon and eBay because I can send coupons to those customers when I ship their products. Its also a great way to get rid of closeouts, or overstocks. I get a 5%+ conversion on my web coupons just from my eBay customers.
The fees are not fun, and eBay can be annoying, but then again, they have built in traffic. My sales have been going down over the last year there, though, because there is a lot of junk merchants selling online that cheapens the whole experience. But I'll take the 5% conversions over nothing... Good luck! Hang in there and make measurable progress each and every day and you'll get a pretty good business going. Just give it time, it is not overnight by any means...
In my area we don't have many good clothing stores so I am usually forced to shop online. These may not be exclusive to just online but none of these have stores in my area. Hope this helps :)