L.A.
It all depends. If the clothing is used, try ebay, if it is handmade by you, try www.Etsy.com (they have a wiki and lots of great advice for new sellers), if you are wanting to wholesale clothing from other designers and want to set up an online clothing boutique... you will need a lot more help!
You will need to register your business name and apply for a resaler's id from the Texas Comptroller. Email or call them here for any extra info on what exactly you need to apply for. You can do that here:
http://www.window.state.tx.us/taxpermit/
http://www.cpa.state.tx.us/taxinfo/taxforms/01-forms.html
There are also classes you can take that are specially made for women starting businesses for a low price, some even free in Fort Worth. They will help you out so much!