You've got two different issues here -- a clothing line & trademarks -- let's tackle one at a time.
RE: Clothing Line
The clothing industry, as you can well imagine, is a very competitive one. To succeed takes knowledge, patience, talent and a bit of luck. As far as the knowledge bit goes, I found some sources that will assist you further.
Associations may be a good avenue to explore as well. These organizations will address many of the thoughts, questions and concerns you'll inevitably have about the clothing industry as well as many you haven't anticipated yet. See the source box for some relevant links.
There's nothing quite like research – the more you know about the clothing industry, the better. There are plenty of free informational resources out there. Check the source box for links to articles.
Also, here are some book titles that may interest you:
* The Official Step-by-Step Guide to Starting a Clothing Line by Jay Arrington,
* The Business of Fashion: Designing, Manufacturing, and Marketing by Leslie Davis Burns
* Fashion for Profit (Revised Edition with Retail Chapter) by Frances Harder
* The Fashion Designer Survival Guide: An Insider's Look at Starting and Running Your Own Fashion Business by Mary Gehlhar
To register a trademark, contact either your Secretary of State for a State trademark or the US Patent & Trademark Office for a Federal trademark.
If you are only conducting business in one state, then a State trademark is most appropriate. If you conduct business in at least 2 states OR between the US & any other country, you can file for a Federal trademark.
Prior to investing your time, money & effort into a name, it is strongly advised that comprehensive research be conducted to ensure that the name you're interested in is truly available.
This entails searching the pending & registered Federal and State trademark files as well as the US National Common-Law files. Then, if clear, you can decide if you would like to file for a Federal or a State trademark.
Hope that helps! I wish you much success & happiness in all your ventures!
http://www.iacde.com/ -- International Association of Clothing Designers & Executives
http://www.apparelsearch.com/association... -- Associations Directory for the Apparel & Fashion Industry
http://ezinearticles.com/?Key-to-Startin... – Key to Starting Your Own Clothing Company by Bradley Johnson
http://ezinearticles.com/?Your-New-Clothing-Company---Apparel-Sourcing&id=69380 – Your New Clothing Company – Apparel Sourcing by Bradley Johnson
http://ezinearticles.com/?Fashion-Marketing-101&id=41674 – Fashion Marketing 101 by Amie Pronowski
http://biztaxlaw.about.com/od/research/a/Sec_of_state.htm -- Contacting your Secretary of State, a listing of links for all states
http://www.uspto.gov – United States Patent & Trademark Office
http://www.uspto.gov/main/trademarks.htm -- USPTO's Main Trademark Page
http://www.nolo.com/resource.cfm/catID/D8932879-DC34-43DF-BF65FC92D55FEE5D/310/274/ -- Nolo's Trademark Resource Center